diff --git a/.github/workflows/ci-build.yml b/.github/workflows/ci-build.yml
index fceb760e6..bacc54b49 100644
--- a/.github/workflows/ci-build.yml
+++ b/.github/workflows/ci-build.yml
@@ -10,10 +10,19 @@ jobs:
   build:
     # Avoiding -latest due to https://github.com/actions/setup-python/issues/162
     runs-on: ubuntu-20.04
-    timeout-minutes: 10
+    timeout-minutes: 15
     strategy:
+      fail-fast: false
       matrix:
-        python-version: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11']
+        python-version:
+          - '3.12'
+          - '3.11'
+          - '3.10'
+          - '3.9'
+          - '3.8'
+          - '3.7'
+          - '3.6'
+          - 'pypy3.10'
     env:
       PYTHON_SLACK_SDK_MOCK_SERVER_MODE: 'threading'
       CI_LARGE_SOCKET_MODE_PAYLOAD_TESTING_DISABLED: '1'
@@ -23,12 +32,13 @@ jobs:
       uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
+        cache: pip
     - name: Install dependencies
       run: |
         pip install -U pip wheel
         pip install -e ".[testing]"
         pip install -e ".[optional]"
-    - name: Run validation
+    - name: Run validation (black/flake8/pytest)
       run: |
         python setup.py validate
     - name: Run tests for SQLAlchemy v1.4 (backward-compatibility)
diff --git a/pytest.ini b/pytest.ini
index c9030f7af..7e88e4b78 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -6,4 +6,5 @@ log_date_format = %Y-%m-%d %H:%M:%S
 filterwarnings =
     ignore:"@coroutine" decorator is deprecated since Python 3.8, use "async def" instead:DeprecationWarning
     ignore:The loop argument is deprecated since Python 3.8, and scheduled for removal in Python 3.10.:DeprecationWarning
+    ignore:slack.* package is deprecated. Please use slack_sdk.* package instead.*:UserWarning
 asyncio_mode = auto
diff --git a/setup.py b/setup.py
index 253332ae6..fd391b1b2 100644
--- a/setup.py
+++ b/setup.py
@@ -17,7 +17,7 @@
     long_description = readme.read()
 
 validate_dependencies = [
-    "pytest>=6.2.5,<7",
+    "pytest>=7.0.1,<8",
     "pytest-asyncio<1",  # for async
     "Flask-Sockets>=0.2,<1",
     "Flask>=1,<2",  # TODO: Flask-Sockets is not yet compatible with Flask 2.x
@@ -25,7 +25,9 @@
     "itsdangerous==1.1.0",  # TODO: Flask-Sockets is not yet compatible with Flask 2.x
     "Jinja2==3.0.3",  # https://github.com/pallets/flask/issues/4494
     "pytest-cov>=2,<3",
-    "flake8>=5,<6",
+    # while flake8 5.x have issues with Python 3.12, flake8 6.x requires Python >= 3.8.1,
+    # so 5.x should be kept in order to stay compatible with Python 3.6/3.7
+    "flake8>=5.0.4,<7",
     # Don't change this version without running CI builds;
     # The latest version may not be available for older Python runtime
     "black==22.8.0",
@@ -123,7 +125,7 @@ def run(self):
             async_source = header + source
             async_source = re.sub("    def ", "    async def ", async_source)
             async_source = re.sub("from asyncio import Future\n", "", async_source)
-            async_source = re.sub("return self.api_call\(", "return await self.api_call(", async_source)
+            async_source = re.sub(r"return self.api_call\(", "return await self.api_call(", async_source)
             async_source = re.sub("-> SlackResponse", "-> AsyncSlackResponse", async_source)
             async_source = re.sub(
                 "from .base_client import BaseClient, SlackResponse",
@@ -132,7 +134,7 @@ def run(self):
             )
             # from slack_sdk import WebClient
             async_source = re.sub(
-                "class WebClient\(BaseClient\):",
+                r"class WebClient\(BaseClient\):",
                 "class AsyncWebClient(AsyncBaseClient):",
                 async_source,
             )
@@ -141,19 +143,19 @@ def run(self):
                 "from slack_sdk.web.async_client import AsyncWebClient",
                 async_source,
             )
-            async_source = re.sub("= WebClient\(", "= AsyncWebClient(", async_source)
+            async_source = re.sub(r"= WebClient\(", "= AsyncWebClient(", async_source)
             async_source = re.sub(
-                " self.files_getUploadURLExternal\(",
+                r" self.files_getUploadURLExternal\(",
                 " await self.files_getUploadURLExternal(",
                 async_source,
             )
             async_source = re.sub(
-                " self.files_completeUploadExternal\(",
+                r" self.files_completeUploadExternal\(",
                 " await self.files_completeUploadExternal(",
                 async_source,
             )
             async_source = re.sub(
-                " self.files_info\(",
+                r" self.files_info\(",
                 " await self.files_info(",
                 async_source,
             )
@@ -163,7 +165,7 @@ def run(self):
                 async_source,
             )
             async_source = re.sub(
-                " _attach_full_file_metadata_async\(",
+                r" _attach_full_file_metadata_async\(",
                 " await _attach_full_file_metadata_async(",
                 async_source,
             )
@@ -178,7 +180,7 @@ def run(self):
                 legacy_source,
             )
             legacy_source = re.sub(
-                "class WebClient\(BaseClient\):",
+                r"class WebClient\(BaseClient\):",
                 "class LegacyWebClient(LegacyBaseClient):",
                 legacy_source,
             )
@@ -187,7 +189,7 @@ def run(self):
                 "from slack_sdk.web.legacy_client import LegacyWebClient",
                 legacy_source,
             )
-            legacy_source = re.sub("= WebClient\(", "= LegacyWebClient(", legacy_source)
+            legacy_source = re.sub(r"= WebClient\(", "= LegacyWebClient(", legacy_source)
             with open(f"{here}/slack_sdk/web/legacy_client.py", "w") as output:
                 output.write(legacy_source)
 
@@ -212,8 +214,10 @@ def run(self):
             "Installing test dependencies ...",
             [sys.executable, "-m", "pip", "install"] + validate_dependencies,
         )
-        self._run("Running black ...", [sys.executable, "-m", "black", f"{here}/slack"])
-        self._run("Running black ...", [sys.executable, "-m", "black", f"{here}/slack_sdk"])
+
+        self._run("Running black for legacy packages ...", [sys.executable, "-m", "black", f"{here}/slack"])
+        self._run("Running black for slack_sdk package ...", [sys.executable, "-m", "black", f"{here}/slack_sdk"])
+
         self._run("Running flake8 for legacy packages ...", [sys.executable, "-m", "flake8", f"{here}/slack"])
         self._run("Running flake8 for slack_sdk package ...", [sys.executable, "-m", "flake8", f"{here}/slack_sdk"])
 
@@ -301,6 +305,7 @@ def run(self):
         "License :: OSI Approved :: MIT License",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
+        "Programming Language :: Python :: 3 :: Only",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
@@ -308,6 +313,8 @@ def run(self):
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
         "Programming Language :: Python :: 3.12",
+        "Programming Language :: Python :: Implementation :: CPython",
+        "Programming Language :: Python :: Implementation :: PyPy",
     ],
     keywords="slack slack-api web-api slack-rtm websocket chat chatbot chatops",
     packages=find_packages(
diff --git a/slack_sdk/oauth/installation_store/internals.py b/slack_sdk/oauth/installation_store/internals.py
index f0b31d394..1c3bfccaf 100644
--- a/slack_sdk/oauth/installation_store/internals.py
+++ b/slack_sdk/oauth/installation_store/internals.py
@@ -1,32 +1,47 @@
-import platform
-import datetime
-
-(major, minor, patch) = platform.python_version_tuple()
-is_python_3_6: bool = int(major) == 3 and int(minor) >= 6
-
-utc_timezone = datetime.timezone.utc
-
-
-def _from_iso_format_to_datetime(iso_datetime_str: str) -> datetime.datetime:
-    if is_python_3_6:
-        elements = iso_datetime_str.split(" ")
-        ymd = elements[0].split("-")
-        hms = elements[1].split(":")
-        return datetime.datetime(
-            int(ymd[0]),
-            int(ymd[1]),
-            int(ymd[2]),
-            int(hms[0]),
-            int(hms[1]),
-            int(hms[2]),
-            0,
-            utc_timezone,
-        )
+import sys
+from datetime import datetime, timezone
+from typing import Type, TypeVar, Union
+
+
+def _from_iso_format_to_datetime(iso_datetime_str: str) -> datetime:
+    if sys.version_info[:2] == (3, 6):
+        format = "%Y-%m-%d %H:%M:%S"
+        if "." in iso_datetime_str:
+            format += ".%f"
+        return datetime.strptime(iso_datetime_str, format).replace(tzinfo=timezone.utc)
     else:
         if "+" not in iso_datetime_str:
             iso_datetime_str += "+00:00"
-        return datetime.datetime.fromisoformat(iso_datetime_str)
+        return datetime.fromisoformat(iso_datetime_str)
 
 
 def _from_iso_format_to_unix_timestamp(iso_datetime_str: str) -> float:
     return _from_iso_format_to_datetime(iso_datetime_str).timestamp()
+
+
+TimestampType = TypeVar("TimestampType", float, int)
+
+
+def _timestamp_to_type(ts: Union[TimestampType, datetime, str], target_type: Type[TimestampType]) -> TimestampType:
+    result: TimestampType
+
+    if isinstance(ts, target_type):
+        # unnecessary type casting makes pytype happy
+        result = target_type(ts)
+
+        # although a type of the timestamp is just checked,
+        # pytype doesn't consider the following line valid:
+        # result = ts
+        # see https://github.com/google/pytype/issues/1012
+
+    elif isinstance(ts, datetime):
+        result = target_type(ts.timestamp())
+    elif isinstance(ts, str):
+        try:
+            result = target_type(ts)
+        except ValueError:
+            result = target_type(_from_iso_format_to_unix_timestamp(ts))
+    else:
+        raise ValueError(f"Unsupported data format for timestamp {ts}")
+
+    return result
diff --git a/slack_sdk/oauth/installation_store/models/bot.py b/slack_sdk/oauth/installation_store/models/bot.py
index 36be38644..e060b8489 100644
--- a/slack_sdk/oauth/installation_store/models/bot.py
+++ b/slack_sdk/oauth/installation_store/models/bot.py
@@ -1,11 +1,8 @@
-import re
 from datetime import datetime  # type: ignore
 from time import time
 from typing import Optional, Union, Dict, Any, Sequence
 
-from slack_sdk.oauth.installation_store.internals import (
-    _from_iso_format_to_unix_timestamp,
-)
+from slack_sdk.oauth.installation_store.internals import _timestamp_to_type
 
 
 class Bot:
@@ -70,30 +67,17 @@ def __init__(
         else:
             self.bot_scopes = bot_scopes
         self.bot_refresh_token = bot_refresh_token
+
         if bot_token_expires_at is not None:
-            if type(bot_token_expires_at) == datetime:
-                self.bot_token_expires_at = int(bot_token_expires_at.timestamp())  # type: ignore
-            elif type(bot_token_expires_at) == str and not re.match("^\\d+$", bot_token_expires_at):
-                self.bot_token_expires_at = int(_from_iso_format_to_unix_timestamp(bot_token_expires_at))
-            else:
-                self.bot_token_expires_at = int(bot_token_expires_at)
+            self.bot_token_expires_at = _timestamp_to_type(bot_token_expires_at, int)
         elif bot_token_expires_in is not None:
             self.bot_token_expires_at = int(time()) + bot_token_expires_in
         else:
             self.bot_token_expires_at = None
+
         self.is_enterprise_install = is_enterprise_install or False
 
-        if type(installed_at) == float:
-            self.installed_at = installed_at  # type: ignore
-        elif type(installed_at) == datetime:
-            self.installed_at = installed_at.timestamp()  # type: ignore
-        elif type(installed_at) == str:
-            if re.match("^\\d+.\\d+$", installed_at):
-                self.installed_at = float(installed_at)
-            else:
-                self.installed_at = _from_iso_format_to_unix_timestamp(installed_at)
-        else:
-            raise ValueError(f"Unsupported data format for installed_at {installed_at}")
+        self.installed_at = _timestamp_to_type(installed_at, float)
 
         self.custom_values = custom_values if custom_values is not None else {}
 
diff --git a/slack_sdk/oauth/installation_store/models/installation.py b/slack_sdk/oauth/installation_store/models/installation.py
index e55cf2b98..09418b36d 100644
--- a/slack_sdk/oauth/installation_store/models/installation.py
+++ b/slack_sdk/oauth/installation_store/models/installation.py
@@ -1,11 +1,8 @@
-import re
 from datetime import datetime  # type: ignore
 from time import time
 from typing import Optional, Union, Dict, Any, Sequence
 
-from slack_sdk.oauth.installation_store.internals import (
-    _from_iso_format_to_unix_timestamp,
-)
+from slack_sdk.oauth.installation_store.internals import _timestamp_to_type
 from slack_sdk.oauth.installation_store.models.bot import Bot
 
 
@@ -100,14 +97,9 @@ def __init__(
         else:
             self.bot_scopes = bot_scopes
         self.bot_refresh_token = bot_refresh_token
+
         if bot_token_expires_at is not None:
-            if type(bot_token_expires_at) == datetime:
-                ts: float = bot_token_expires_at.timestamp()  # type: ignore
-                self.bot_token_expires_at = int(ts)
-            elif type(bot_token_expires_at) == str and not re.match("^\\d+$", bot_token_expires_at):
-                self.bot_token_expires_at = int(_from_iso_format_to_unix_timestamp(bot_token_expires_at))
-            else:
-                self.bot_token_expires_at = bot_token_expires_at  # type: ignore
+            self.bot_token_expires_at = _timestamp_to_type(bot_token_expires_at, int)
         elif bot_token_expires_in is not None:
             self.bot_token_expires_at = int(time()) + bot_token_expires_in
         else:
@@ -120,14 +112,9 @@ def __init__(
         else:
             self.user_scopes = user_scopes
         self.user_refresh_token = user_refresh_token
+
         if user_token_expires_at is not None:
-            if type(user_token_expires_at) == datetime:
-                ts: float = user_token_expires_at.timestamp()  # type: ignore
-                self.user_token_expires_at = int(ts)
-            elif type(user_token_expires_at) == str and not re.match("^\\d+$", user_token_expires_at):
-                self.user_token_expires_at = int(_from_iso_format_to_unix_timestamp(user_token_expires_at))
-            else:
-                self.user_token_expires_at = user_token_expires_at  # type: ignore
+            self.user_token_expires_at = _timestamp_to_type(user_token_expires_at, int)
         elif user_token_expires_in is not None:
             self.user_token_expires_at = int(time()) + user_token_expires_in
         else:
@@ -143,17 +130,8 @@ def __init__(
 
         if installed_at is None:
             self.installed_at = datetime.now().timestamp()
-        elif type(installed_at) == float:
-            self.installed_at = installed_at  # type: ignore
-        elif type(installed_at) == datetime:
-            self.installed_at = installed_at.timestamp()  # type: ignore
-        elif type(installed_at) == str:
-            if re.match("^\\d+.\\d+$", installed_at):
-                self.installed_at = float(installed_at)
-            else:
-                self.installed_at = _from_iso_format_to_unix_timestamp(installed_at)
         else:
-            raise ValueError(f"Unsupported data format for installed_at {installed_at}")
+            self.installed_at = _timestamp_to_type(installed_at, float)
 
         self.custom_values = custom_values if custom_values is not None else {}
 
diff --git a/tests/slack_sdk/oauth/installation_store/test_internals.py b/tests/slack_sdk/oauth/installation_store/test_internals.py
index 5d0f3d435..948aa8c15 100644
--- a/tests/slack_sdk/oauth/installation_store/test_internals.py
+++ b/tests/slack_sdk/oauth/installation_store/test_internals.py
@@ -1,7 +1,11 @@
+import sys
 import unittest
+from datetime import datetime, timezone
+
+import pytest
 
 from slack_sdk.oauth.installation_store import Installation, FileInstallationStore
-from slack_sdk.oauth.installation_store.internals import _from_iso_format_to_datetime
+from slack_sdk.oauth.installation_store.internals import _from_iso_format_to_datetime, _timestamp_to_type
 
 
 class TestFile(unittest.TestCase):
@@ -14,3 +18,29 @@ def tearDown(self):
     def test_iso_format(self):
         dt = _from_iso_format_to_datetime("2021-07-14 08:00:17")
         self.assertEqual(dt.timestamp(), 1626249617.0)
+
+
+@pytest.mark.parametrize('ts,target_type,expected_result', [
+    (1701209097, int, 1701209097),
+    (datetime(2023, 11, 28, 22, 9, 7, tzinfo=timezone.utc), int, 1701209347),
+    ("1701209605", int, 1701209605),
+    ("2023-11-28 22:11:19", int, 1701209479),
+    (1701209998.3429494, float, 1701209998.3429494),
+    (datetime(2023, 11, 28, 22, 20, 25, 262571, tzinfo=timezone.utc), float, 1701210025.262571),
+    ("1701210054.4672053", float, 1701210054.4672053),
+    ("2023-11-28 22:21:14.745556", float, 1701210074.745556),
+])
+def test_timestamp_to_type(ts, target_type, expected_result):
+    result = _timestamp_to_type(ts, target_type)
+    assert result == expected_result
+
+
+def test_timestamp_to_type_invalid_str():
+    match = "Invalid isoformat string" if sys.version_info[:2] > (3, 6) else "time data .* does not match format"
+    with pytest.raises(ValueError, match=match):
+        _timestamp_to_type('not-a-timestamp', int)
+
+
+def test_timestamp_to_type_unsupported_format():
+    with pytest.raises(ValueError, match="Unsupported data format"):
+        _timestamp_to_type({}, int)
diff --git a/tests/slack_sdk_async/socket_mode/test_interactions_websockets.py b/tests/slack_sdk_async/socket_mode/test_interactions_websockets.py
index a935c0f3c..b1b7c1652 100644
--- a/tests/slack_sdk_async/socket_mode/test_interactions_websockets.py
+++ b/tests/slack_sdk_async/socket_mode/test_interactions_websockets.py
@@ -90,7 +90,9 @@ async def socket_mode_listener(
             expected.sort()
 
             count = 0
-            while count < 10 and len(received_messages) < len(expected):
+            while count < 10 and (
+                len(received_messages) < len(expected) or len(received_socket_mode_requests) < len(socket_mode_envelopes)
+            ):
                 await asyncio.sleep(0.2)
                 count += 0.2