tests still rely on the removed stdlib modules asyncore/asynchat

[arhadthedev]

Reference: PEP 594 – Removing dead batteries from the standard library.

This issue tracks removal of the following modules:

• Demote from the public API into the test helpers
  • [x] smtpd (gh-93246)
  • [x] asynchat and asyncore (gh-96580)
• Port tests onto relevant analogs, step by step
  • smtpd
  • [ ] test.test_smtplib
  • asynchat and asyncore
  • [ ] test.test_ftplib
  • [ ] test.test_logging
  • [ ] test.test_poplib
    • try to give priority for gh-1864
  • [ ] test.test_smtplib
  • [ ] test.test_ssl
  • [ ] test.libregrtest.save_env
• Remove the remnescants
  • [ ] Lib/test/smtpd.py, PCbuild/*
  • [ ] Lib/test/support/async(hat|ore).py, PCbuild/*
  • [ ] Dead links from What's New and release notes

Related issues:

• gh-72719 (created 3 years before PEP 594 was written)
• gh-91171 (I lost authorship there during the porting from the bugs.python.org)

[JelleZijlstra]

This requires finishing #1864 (which I need to get back to).

[arhadthedev]

Asyncore-based test servers from test_ftp and test_logging cannot be ported to asyncio, the tested clients are blocking. However, instead of writing a server thread class from scratch, it would be nice to reuse test.support.threading_helper.Server from gh-92475.

[iritkatriel]

Changing the title to reflect what still needs to be done.