The previous code was using the legacy Group Chat 1.0 (GC1.0)
protocol to join [XEP-0045 Multi User Chats][1]. The legacy
protocol is described in § 7.2.1 of that document, the current
protocol is described in § 7.2.2.
The legacy protocol has not been in active use for more than ten
years, and servers are fading out support for it because its
presence causes issues (see for example the lengthy discussion
in [2], particularly the part starting with "A MUC
misunderstanding a presence update for a GC1.0 join").
The effect of servers fading out GC1.0 is that jabber.py cannot
send messages to rooms on servers which have done that step any
more.
This commit implements the modern join protocol, restoring
functionality. The modern join protocol is, to my knowledge,
supported by all XMPP servers which are still in use.
Prosody 0.11 is an example of a server implementation which does
not support GC1.0 anymore.
[1]: https://xmpp.org/extensions/xep-0045.html
[2]: https://mail.jabber.org/pipermail/standards/2017-October/033501.html
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