Tweak TLS version

It seems, that the sleekxmpp defaults to TLS 1.0, it is needed to tweak it for modern servers, e.g. with this in XmppBot's __init__() (import ssl of course):

self.ssl_version = ssl.PROTOCOL_TLS

I got connected with TLS 1.3, without it i got "Unsupported protocol" in prosody log and infinite loop with misleading report about "Invalid certificate trust chain"...