bpo-44498: Issue a deprecation warning on asynchat, asyncore and smtpd import (#26882)

* Issue a deprecation warning on smtpd import

* Also issue DeprecationWarnings for asynchat and asyncore

* Fix some tests

* test___all__ requires the word 'module' or 'package' in the deprecation
  warning text, so add those to smtpd, asynchat, and asyncore.
* In test_support, use pprint now instead of asyncore as the landmark.

* Add What's New

* Use ..deprecated::

* Use ..deprecated::

* Update Lib/smtpd.py

Co-authored-by: Miro Hrončok <miro@hroncok.cz>

* Update Doc/library/smtpd.rst

Co-authored-by: Miro Hrončok <miro@hroncok.cz>

* Import async{hat,ore} after the DeprecationWarning for this module

Co-authored-by: Miro Hrončok <miro@hroncok.cz>