Support for TLS 1.2 only servers

[lilydjwg]

This line specifies to only use TLS 1.0:

pyxmpp2/pyxmpp2/streamtls.py

Line 181 in 14a40a3

ssl_version = ssl.PROTOCOL_TLSv1,

Which is old and people are dropping support for it. For whatever reasons, my Prosody on Debian buster has stopped supporting TLS 1.0 too and my bots are no longer able to connect to it.

Using ssl.PROTOCOL_TLS will have broader support but it requires Python >= 3.6. Using ssl.PROTOCOL_TLSv1_2 will support Python 3.4 but is not future-proof.