Test suite's otr-bot.py has obsolete dependencies

It’s written in Python 2, which is not supposed to be included in Bullseye.

Moreover, it depends on:

python-jabberbot: still in Buster but won’t be in Bullseye; the version in Debian is Python 2 only
python-xmpp: antique CVS snapshot; orphaned; on life support since years (e.g. #9074 (closed)); the version in Debian is Python 2 only and therefore will likely not make it into Bullseye

So it seems like this will need to be rewritten using maintained Python 3 modules once we want to run the test suite on Bullseye.

Original created by @intrigeri on 17031 (Redmine)

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