bpo-28533: Remove asyncore, asynchat, smtpd modules

[vstinner]

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

https://bugs.python.org/issue28533

[vstinner]

A year ago @aeros worked on a patch: https://github.com/aeros/cpython/tree/remove-asycore-asynchat-smtpd

Kyle mentioned the PEP 594. I prefer to not mention it since this PEP is still a draft. IMO the deprecation and linked to bpo issues is enough to justify the removals.

cc @tiran

[orsenthil]

LGTM. Thank you, @vstinner :)

[gvanrossum]

It's time for these long-deprecated packages to leave.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @tiran for commit 1bf97aa 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[vstinner]

I looked at buildbot failures:

• AMD64 Windows10: unrelated, test_importlib failed. It looks like a Git clone issue. Not sure why it occurs on "PR" builds.
• AMD64 Arch Linux Asan PR: unrelated, https://bugs.python.org/issue45797

[vstinner]

Merged. I prefer to merge such change early in the 3.11 devcycle, so we can revert it if it breaks too many projects.

Thanks for the reviews and approvals!

[Karlson2k]

In the meantime the suggested replacement supports Python versions up to 3.9. Python 3.10 and later is not supported.
There is no single commit in the past 14 months in the repo.
Python 3.11 will be released without working replacement. The suggested replacement is not compatible with 3.11.