util.inspect.getdoc doesn't accept a method decorated by a class with __get__ method

[fbessou]

Describe the bug

When decorating a method with the python implementation of classmethod described here sphinx crashes complaining that the decorator has no attribute __func__

Here is a copy of the ClassMethod definition:

class ClassMethod:
    "Emulate PyClassMethod_Type() in Objects/funcobject.c"

    def __init__(self, f):
        self.f = f

    def __get__(self, obj, cls=None):
        if cls is None:
            cls = type(obj)
        if hasattr(type(self.f), '__get__'):
            return self.f.__get__(cls)
        return MethodType(self.f, cls)

How to Reproduce

$ git clone https://github.com/fbessou/sphinx-getitem
$ cd sphinx-getitem
$ pip install sphinx  # or use a virtualenv
$ cd doc
$ make html

The build crashes on the following error:

  File "<ENV>/lib/python3.7/site-packages/sphinx/util/inspect.py", line 853, in getdoc
    return getdoc(meth.__func__)
AttributeError: 'ClassMethod' object has no attribute '__func__'

Expected behavior

Sphinx shouldn't crash and the documented method should appear in the built documentation.

Your project

https://github.com/fbessou/sphinx-getitem

Screenshots

No response

OS

Linux (Debian Buster)

Python version

3.7

Sphinx version

4.2.0

Sphinx extensions

sphix.ext.autodoc

Extra tools

No response

Additional context

No response