[3.9] bpo-42073: allow classmethod to wrap other classmethod-like descriptors.

[eriknw]

bpo-19072 (#8405) allows classmethod to wrap other descriptors, but this does not work when the wrapped descriptor mimics classmethod. The current PR fixes this.

In Python 3.8 and before, one could create a callable descriptor such that this works as expected (see Lib/test/test_decorators.py for examples):

class A:
    @myclassmethod
    def f1(cls):
        return cls

    @classmethod
    @myclassmethod
    def f2(cls):
        return cls

In Python 3.8 and before, A.f2() return A. Currently in Python 3.9, it returns type(A). This PR make A.f2() return A again.

As of #8405, classmethod calls obj.__get__(type) if obj has __get__. This allows one to chain @classmethod and @property together. When using classmethod-like descriptors, it's the second argument to __get__--the owner or the type--that is important, but this argument is currently missing. Since it is None, the "owner" argument is assumed to be the type of the first argument, which, in this case, is wrong (we want A, not type(A)).

This PR updates classmethod to call obj.__get__(type, type) if obj has __get__.

This PR is targeting Python 3.9 branch, because I think this is a bug fix. I can also target master (3.10).

I'm not really sure where to add a note of this change.

https://bugs.python.org/issue42073

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[ambv]

Oops, changing the base of the pull request didn't do what I thought it would and reverting back to 3.9 leaves the mess as is. I'll recreate this pull request on main from scratch.