Cleanup SSL deprecation warnings under Python 3.10 and newer (#706)

Prior to this commit python would produce these warnings when using
kazoo with ssl:

  /path/to/venv/lib/python3.11/site-packages/kazoo/handlers/utils.py:225: DeprecationWarning: ssl.PROTOCOL_TLS is deprecated
context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)

The reason for this is that ssl.PROTOCOL_SSLv23 is an alias for
ssl.PROTOCOL_TLS and ssl.PROTOCOL_TLS is deprecated since Python 3.10.

ssl.PROTOCOL_TLS was replaced with ssl.PROTOCOL_TLS_CLIENT and
ssl.PROTOCOL_TLS_SERVER. In kazoo's case we switch to
ssl.PROTOCOL_TLS_CLIENT as kazoo is acting as an ssl client to zookeeper
servers.

There are a few things to note. PROTOCOL_TLS_CLIENT enables
context.check_hostname. We explicitly set this to False as this is
required to set ssl.CHECK_NONE which kazoo supports, and not everyone
may be using SSL certs with proper hostnames configured. For example if
making connections to an IP address rather than a name and the certs
don't have IP addrs in their altnames. This ensures backward
compatibility with these use cases. Changing this should be done in a
separate change and should likely be made configurable like
verify_certs.

Finally, while we are at it we replace ssl.CERT_OPTIONAL with
ssl.CERT_REQUIRED as they are equivalent in a client context. This
allows us to delete some code.

Python documents all of these behaviors as being present since Python
3.6. Kazoo requires Python 3.7 or newer which should make this safe.

kazoo/handlers/utils.py

@@ -225,7 +225,7 @@ def create_tcp_connection(
 
         if use_ssl:
             # Disallow use of SSLv2 and V3 (meaning we require TLSv1.0+)
-            context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
+            context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
 
             if options is not None:
                 context.options = options
@@ -238,15 +238,17 @@ def create_tcp_connection(
 
             # Load default CA certs
             context.load_default_certs(ssl.Purpose.SERVER_AUTH)
+            # We must set check_hostname to False prior to setting
+            # verify_mode to CERT_NONE.
+            # TODO: Make hostname verification configurable as some users may
+            # elect to use it.
+            context.check_hostname = False
             context.verify_mode = (
-                ssl.CERT_OPTIONAL if verify_certs else ssl.CERT_NONE
+                ssl.CERT_REQUIRED if verify_certs else ssl.CERT_NONE
             )
             if ca:
                 context.load_verify_locations(ca)
             if certfile and keyfile:
-                context.verify_mode = (
-                    ssl.CERT_REQUIRED if verify_certs else ssl.CERT_NONE
-                )
                 context.load_cert_chain(
                     certfile=certfile,
                     keyfile=keyfile,