bpo-28533: Remove asyncore, asynchat, smtpd modules (GH-29521)

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