test-patchbomb-tls.t would fail with:
  .../hg/tests/dummysmtpd.py:6: DeprecationWarning: The asyncore module is deprecated and will be removed in Python 3.12. The recommended replacement is asyncio
    import asyncore
  .../hg/tests/dummysmtpd.py:8: DeprecationWarning: The smtpd module is deprecated and unmaintained and will be removed in Python 3.12.  Please see aiosmtpd (https://aiosmtpd.readthedocs.io/) for the recommended replacement.
    import smtpd

The recommended migration path to the standalone asiosmtpd would be overkill.

The tests do not need a full smtp server - we can just use a very simple mock
hack to preserve the existing test coverage.