Document PEP 387 Soft Deprecation

[vstinner]

Documentation

Document https://peps.python.org/pep-0387/#soft-deprecation

Linked PRs

• gh-106535: Document PEP 387 Soft Deprecation #106536
• gh-106535: Soft deprecate the getopt module #105735
• gh-106535: Document soft deprecations in What's New In Python 3.13 #106859

[ncoghlan]

Note that PEP 389 specifically stated that getopt was not deprecated: https://peps.python.org/pep-0389/#updates-to-getopt-documentation

Follow-up on this in #126225