bpo-42073: allow classmethod to wrap other classmethod-like descripto…

[sthagen]

…rs (python#27115)

Patch by Erik Welch.

bpo-19072 (python#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 python#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__.

Co-authored-by: Erik Welch erik.n.welch@gmail.com