gh-84644: Fix singledispatch annotation parsing

[johnslavik]

A very practical but more general approach than GH-140255 to fixing annotation parsing in functools.singledispatch and functools.singledispatchmethod.

Aims to fix issues GH-84644 and GH-130827.

It can be broken if one uses a user-defined alternative implementation of staticmethod or something analogous.
Will break incorrect but working registrees. I haven't investigated stripping Annotated typeforms yet.

Consulting a test which fails with this fix at https://github.com/python/cpython/pull/130309/changes#r2663516538 -- I think that the test is wrong.

[johnslavik]

I was asked to filter out tests that were helpful at the time of writing this code with detecting backward compatibility problems, but already pass on main. If we're interested in increasing coverage, they'll be recoverable from a commit that removes them.

I'll do it in the next few days.

In the meantime, the implementation on its own can be reviewed.

[JelleZijlstra]

Looks reasonable to me!

[johnslavik]

I'll do it in the next few days.

Done now.

Ready for a proper review.