Add support for singledispatchmethod class methods in MethodDocumenter

[picnixz]

Fix #11278. Fix #11531.

Details:

• Add :classmethod: directive option for @classmethod combined with @singledispatchmethod.
• Add tests.

~~There is some decision making to doc for this feature concerning the compatibility for Python versions up to 3.9 (excluded), so I want to get some feedback before merging.~~

~~Prior to Python 3.9~~

Legacy comment

If a method is decorated with @singledispatchmethod and @classmethod, the MethodDocumenter miserably fails to extract the correct signature or docstring. However, in order to properly extract the signature or the docstring, this would require a lot of changes in the sphinx.util.inspect or sphinx.ext.autodoc module which may break other things.

Therefore, I think it would be better to say "autodocumenting a singledispatched classmethod works assuming that Sphinx is running Python 3.9 or later".

[picnixz]

Since we are dropping 3.8 support, I'll clean this PR when I've got time.

[picnixz]

I wanted to rename some branch but it closed the PR. So I'll stick with an old name.

[picnixz]

I've had this PR for a long time and nothing has ever changed, so let's merge this one now.