some versions of python do not react to stream:features starttls required

[George Rey]

on python 3.10 I get normal, expected behaviour during connection initiation:

DEBUG    RECV: <stream:features xmlns="http://etherx.jabber.org/streams"><starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls"><required /></starttls></stream:features>

DEBUG    SEND: <starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls" />
DEBUG    RECV: <proceed xmlns="urn:ietf:params:xml:ns:xmpp-tls" />
DEBUG    Starting TLS   

and rest of the connection proceeds normally, and my tests pass

python 3.7.0, pypy 3.7.12, and PyPy 7.3.8(python 3.9.10)  I get, and this is the last debug message before hanging indefinitely:

``DEBUG  RECV: <stream:features xmlns="http://etherx.jabber.org/streams"><starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls"><required /></starttls></stream:features>``

And that's the end of the conversation. 

The `sleekxmpp` never responds with "SEND: <starttls(...)" and the negotiation hangs up.

There is no exception, no error, nothing. . . . how do I debug this?

I have tried to follow the debugger, and it seems that in

starttls.py: 57

            self.xmpp.send(stanza.STARTTLS())

does get executed,

however, somehow this stanza never gets actually sent out

any hints please?