Unnecessary logging of a Traceback from raise StopIteration.

When StopIteration is raised during workflow processing, A log entry like

Jan 29 03:05:03 2023 (3196892) deque: do_confirm_verify
Traceback (most recent call last):
  File "/opt/mailman/mm/venv/lib/python3.8/site-packages/mailman/app/workflow.py", line 69, in __next__
    return step()
  File "/opt/mailman/mm/venv/lib/python3.8/site-packages/mailman/app/subscriptions.py", line 392, in _step_send_invitation
    raise StopIteration
StopIteration

is created. This is normal processing and shouldn't be logged or at least not in a way that makes it look like an error.