Client: Unregister handlers on cleanup()

Fixes #10083
3 jobs for master in 2 minutes and 39 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #17161
run-appdata

00:00:04

failed #17160
run-tests

00:02:34

 
  Build
skipped #17162
run-build
 
Name Stage Failure
failed
run-tests Test
OK
$ codespell -I codespell.conf --skip="*__pycache__*,gajim/data/icons,gajim/data/sounds,gajim/data/emoticons" gajim
/usr/lib/python3.8/codecs.py:905: RuntimeWarning: line buffering (buffering=1) isn't supported in binary mode, the default buffer size will be used
file = builtins.open(filename, mode, buffering)
gajim/common/client.py:535: befor ==> before
gajim/common/client.py:538: befor ==> before
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit status 1