gh-93243
This PR is required to reduce diffs of the following porting (no need to either maintain documentation and tests consistent with each porting step, or try to port everything and remove smtpd in a single PR).
Automerge-Triggered-By: GH:warsaw
Remove the asyncore and asynchat modules, deprecated in Python
3.6: use the asyncio module instead.
Remove the smtpd module, deprecated in Python 3.6: the aiosmtpd
module can be used instead, it is based on asyncio.
* Remove asyncore, asynchat and smtpd documentation
* Remove test_asyncore, test_asynchat and test_smtpd
* Rename Lib/asynchat.py to Lib/test/support/_asynchat.py
* Rename Lib/asyncore.py to Lib/test/support/_asyncore.py
* Rename Lib/smtpd.py to Lib/test/support/_smtpd.py
* Remove DeprecationWarning from private _asyncore, _asynchat and
_smtpd modules
* _smtpd: remove deprecated properties
Patch by Milan Oberkirch, developed as part of his 2014 GSOC project.
Note that this also fixes a bug in mock_socket ('getpeername' was returning a
simple string instead of the tuple required for IPvX protocols), a bug in
DebugServer with respect to handling binary data (should have been fixed when
decode_data was introduced, but wasn't found until this patch was written),
and a long-standing bug in DebugServer (it was printing an extra blank line at
the end of the displayed message text).
