Dispatcher: Fix stanza handler registration

2 jobs for typing in 7 seconds
Status Job ID Name Coverage
  Test
failed #16535
run-test

00:00:07

 
  Build
skipped #16536
run-build
 
Name Stage Failure
failed
run-test Test
  File "/home/gitlab-runner/builds/c0eaa480/0/linkmauve/python-nbxmpp/test/unit/test_tune.py", line 55, in test_tune_parsing
self.dispatcher.register_handler(
TypeError: register_handler() argument after * must be an iterable, not StanzaHandler

----------------------------------------------------------------------
Ran 32 tests in 0.117s

FAILED (errors=10)
ERROR: Job failed: exit status 1