bpo-37203: Correct classmethod Python emulation in the Descriptor HowTo Guide

[geryogam]

With the current Python equivalent ClassMethod implementation of classmethod given in Raymond Hettinger's Descriptor HowTo Guide, the following code snippet:

class A:
    @ClassMethod
    def f(cls, *, x):
        pass

print(A.f)
A.f(x=3)

prints:

<function ClassMethod.__get__..newfunc at 0x106b76268>

and raises:

TypeError: newfunc() got an unexpected keyword argument 'x'

instead of only printing:

<bound method A.f of <class '__main__.A'>>

like the @classmethod decorator would do.

So the ClassMethod implementation fails in two regards:

• it does not return a bound method to a class;
• it does not handle keyword-only parameters.

With this PR ClassMethod will correctly emulate classmethod. This approach (types.MethodType) is already used in the Python equivalent Function implementation of functions given earlier in the same guide.

https://bugs.python.org/issue37203