[3.10] bpo-47022: Document asynchat, asyncore and smtpd removals in 3.12 (GH-31891) (#31997)

Document the deprecation of asyncore, asynchat, and smtpd with a slated removal in Python 3.12 thanks to PEP 594..
(cherry picked from commit 77473846439b8a3eae66de1a1cfe931619f38513)

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>