[3.10] bpo-44498: Issue a deprecation warning on asynchat, asyncore and smtpd import (GH-26882) (GH-26904)

* 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>
(cherry picked from commit 8488b85c63)


Co-authored-by: Barry Warsaw <barry@python.org>

Automerge-Triggered-By: GH:warsaw

Lib/smtpd.py

@@ -83,8 +83,6 @@ import errno
 import getopt
 import time
 import socket
-import asyncore
-import asynchat
 import collections
 from warnings import warn
 from email._header_value_parser import get_addr_spec, get_angle_addr
@@ -94,6 +92,19 @@ __all__ = [
     "MailmanProxy",
 ]
 
+warn(
+    'The smtpd module is deprecated and unmaintained.  Please see aiosmtpd '
+    '(https://aiosmtpd.readthedocs.io/) for the recommended replacement.',
+    DeprecationWarning,
+    stacklevel=2)
+
+
+# These are imported after the above warning so that users get the correct
+# deprecation warning.
+import asyncore
+import asynchat
+
+
 program = sys.argv[0]
 __version__ = 'Python SMTP proxy version 0.3'