Use largest version of moto possible

To stop testing with old versions of moto, unpin it from version 3, to
the latest, depending on the Python version being used. Support both
version 4 and 5 in the test suite.

requirements/testing.txt

@@ -14,4 +14,7 @@ click==8.0.4  # black is affected by https://github.com/pallets/click/issues/222
 psutil>=5,<6
 #  used only under slack_sdk/*_store
 boto3<=2
-moto>=3,<4  # For AWS tests
+# For AWS tests
+moto==4.0.13; python_version=="3.6"
+moto<5; python_version=="3.7"
+moto<6

tests/slack_sdk/oauth/installation_store/test_amazon_s3.py

@@ -1,23 +1,27 @@
 import unittest
 
 import boto3
-from moto import mock_s3
+
+try:
+    from moto import mock_aws
+except ImportError:
+    from moto import mock_s3 as mock_aws
 from slack_sdk.oauth.installation_store import Installation
 from slack_sdk.oauth.installation_store.amazon_s3 import AmazonS3InstallationStore
 
 
 class TestAmazonS3(unittest.TestCase):
-    mock_s3 = mock_s3()
+    mock_aws = mock_aws()
     bucket_name = "test-bucket"
 
     def setUp(self):
-        self.mock_s3.start()
+        self.mock_aws.start()
         s3 = boto3.resource("s3")
         bucket = s3.Bucket(self.bucket_name)
         bucket.create(CreateBucketConfiguration={"LocationConstraint": "af-south-1"})
 
     def tearDown(self):
-        self.mock_s3.stop()
+        self.mock_aws.stop()
 
     def build_store(self) -> AmazonS3InstallationStore:
         return AmazonS3InstallationStore(

tests/slack_sdk/oauth/state_store/test_amazon_s3.py

@@ -1,22 +1,26 @@
 import unittest
 
 import boto3
-from moto import mock_s3
+
+try:
+    from moto import mock_aws
+except ImportError:
+    from moto import mock_s3 as mock_aws
 from slack_sdk.oauth.state_store.amazon_s3 import AmazonS3OAuthStateStore
 
 
 class TestAmazonS3(unittest.TestCase):
-    mock_s3 = mock_s3()
+    mock_aws = mock_aws()
     bucket_name = "test-bucket"
 
     def setUp(self):
-        self.mock_s3.start()
+        self.mock_aws.start()
         s3 = boto3.resource("s3")
         bucket = s3.Bucket(self.bucket_name)
         bucket.create(CreateBucketConfiguration={"LocationConstraint": "af-south-1"})
 
     def tearDown(self):
-        self.mock_s3.stop()
+        self.mock_aws.stop()
 
     def test_instance(self):
         store = AmazonS3OAuthStateStore(