[3.9] bpo-39679: Fix `singledispatchmethod` `classmethod`/`staticmethod` bug (GH...

[3.9] bpo-39679: Fix `singledispatchmethod` `classmethod`/`staticmethod` bug (GH-29087)

This commit fixes a bug in the 3.9 branch where stacking
`@functools.singledispatchmethod` on top of `@classmethod` or `@staticmethod`
caused an exception to be raised if the method was registered using
type-annotations rather than `@method.register(int)`. Tests for this scenario
were added to the 3.11 and 3.10 branches in #29034 and #29072; this commit
also backports those tests to the 3.9 branch.

Co-authored-by: Yurii Karabas <1998uriyyo@gmail.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>