PEP 594 – Removing dead batteries from the standard library

As mentioned in the SC acceptance:

> One thing we’d like to see happen while implementing it: Document the status of the modules being deprecated and removed and backport those deprecation updates to older CPython branch documentation (at least back to 3.9). That gets the notices in front of more people who may use the docs for their specific Python version.

So let's update documentation and deprecation warnings to make clear in which release they're to be removed.

This first one is just for the three modules (asynchat, asyncore and smtpd) slated for removal soonest, in 3.12:

https://peps.python.org/pep-0594/#deprecated-modules