Replace deprecated constant PROTOCOL_SSLv23 with PROTOCOL_TLS

[AaronAtDuo]

Per https://docs.python.org/3/library/ssl.html:

ssl.PROTOCOL_TLS
Selects the highest protocol version that both the client and server support. Despite the name, this option can select both “SSL” and “TLS” protocols.

New in version 3.6.

ssl.PROTOCOL_SSLv23
Alias for [PROTOCOL_TLS](https://docs.python.org/3/library/ssl.html#ssl.PROTOCOL_TLS).

Deprecated since version 3.6: Use [PROTOCOL_TLS](https://docs.python.org/3/library/ssl.html#ssl.PROTOCOL_TLS) instead.

so there should be no functional change with this update, which we can make now that we don't support 3.5 or earlier.

[mbish]

ssl.PROTOCOL_TLS
...
Deprecated since version 3.10

Should we just jump straight to ssl.PROTOCOL_TLS_CLIENT?

[AaronAtDuo]

with PROTOCOL_TLS_CLIENT a couple tests fail. After looking into it, I know why, but fixing it technically changes some behavior. So I want to have that as a separate PR / conversation and keep this to solely removing the super-deprecated constant.

[mbish]

Okay that's fine then; we can leave it for later

[nstojcevich]

LGTM