classmethod tests fail with Python 3.13 (Python reverted to pre-3.9 behavior)

Similarily to https://github.com/GrahamDumpleton/wrapt/issues/160 the tests now fail with Python 3.13.0a4.

To reproduce, I did:

$ git clone git@github.com:GrahamDumpleton/wrapt.git
$ cd wrapt/
[wrapt (develop)]$ tox -e py313
py313: failed with unable to determine pip install command: attempting to parse '' into a command failed
  py313: FAIL code 1 (0.20 seconds)
  evaluation failed :( (0.25 seconds)

I did not understand the error, so I changed:

diff --git a/setup.cfg b/setup.cfg
index 9c282c9..9c391b3 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -91,8 +91,6 @@ python =
 deps =
   coverage
   pytest
-install_command =
-  py311,py311-{without,install,disable}-extensions: python -m pip install --no-binary coverage {opts} {packages}
 commands =
   python -m coverage run --rcfile {toxinidir}/setup.cfg -m pytest -v {posargs} {toxinidir}/tests
 setenv =

And continued:

[wrapt (develop *)]$ tox -e py313
.pkg: _optional_hooks> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: get_requires_for_build_sdist> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: get_requires_for_build_wheel> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: prepare_metadata_for_build_wheel> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: build_sdist> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
py313: install_package> python -I -m pip install --force-reinstall --no-deps .../wrapt/.tox/.tmp/package/2/wrapt-1.16.0.tar.gz
py313: commands[0]> python -m coverage run --rcfile .../wrapt/setup.cfg -m pytest -v .../wrapt/tests
============================= test session starts ==============================
platform linux -- Python 3.13.0a4, pytest-8.0.2, pluggy-1.4.0 -- .../wrapt/.tox/py313/bin/python
cachedir: .tox/py313/.pytest_cache
rootdir: .../wrapt
configfile: setup.cfg
collecting ... collected 438 items

...

=================================== FAILURES ===================================
_____________ TestCallingOuterClassMethod.test_class_call_function _____________

self = <test_outer_classmethod.TestCallingOuterClassMethod testMethod=test_class_call_function>

    def test_class_call_function(self):
        # Test calling classmethod. Prior to Python 3.9, the instance
        # and class passed to the wrapper will both be None because our
        # decorator is surrounded by the classmethod decorator. The
        # classmethod decorator doesn't bind the method and treats it
        # like a normal function, explicitly passing the class as the
        # first argument with the actual arguments following that. This
        # was only finally fixed in Python 3.9. For more details see:
        # https://bugs.python.org/issue19072

        _args = (1, 2)
        _kwargs = {'one': 1, 'two': 2}

        @wrapt.decorator
        def _decorator(wrapped, instance, args, kwargs):
            if PYXY < (3, 9):
                self.assertEqual(instance, None)
                self.assertEqual(args, (Class,)+_args)
            else:
                self.assertEqual(instance, Class)
                self.assertEqual(args, _args)

            self.assertEqual(kwargs, _kwargs)
            self.assertEqual(wrapped.__module__, _function.__module__)
            self.assertEqual(wrapped.__name__, _function.__name__)

            return wrapped(*args, **kwargs)

        @_decorator
        def _function(*args, **kwargs):
            return args, kwargs

        class Class(object):
            @classmethod
            @_decorator
            def _function(cls, *args, **kwargs):
                return (args, kwargs)

>       result = Class._function(*_args, **_kwargs)

tests/test_outer_classmethod.py:160: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_outer_classmethod.py:141: in _decorator
    self.assertEqual(instance, Class)
E   AssertionError: None != <class 'test_outer_classmethod.TestCallin[54 chars]ass'>
___________ TestCallingOuterClassMethod.test_instance_call_function ____________

self = <test_outer_classmethod.TestCallingOuterClassMethod testMethod=test_instance_call_function>

    def test_instance_call_function(self):
        # Test calling classmethod via class instance. Prior to Python
        # 3.9, the instance and class passed to the wrapper will both be
        # None because our decorator is surrounded by the classmethod
        # decorator. The classmethod decorator doesn't bind the method
        # and treats it like a normal function, explicitly passing the
        # class as the first argument with the actual arguments
        # following that. This was only finally fixed in Python 3.9. For
        # more details see: https://bugs.python.org/issue19072

        _args = (1, 2)
        _kwargs = {'one': 1, 'two': 2}

        @wrapt.decorator
        def _decorator(wrapped, instance, args, kwargs):
            if PYXY < (3, 9):
                self.assertEqual(instance, None)
                self.assertEqual(args, (Class,)+_args)
            else:
                self.assertEqual(instance, Class)
                self.assertEqual(args, _args)

            self.assertEqual(kwargs, _kwargs)
            self.assertEqual(wrapped.__module__, _function.__module__)
            self.assertEqual(wrapped.__name__, _function.__name__)

            return wrapped(*args, **kwargs)

        @_decorator
        def _function(*args, **kwargs):
            return args, kwargs

        class Class(object):
            @classmethod
            @_decorator
            def _function(cls, *args, **kwargs):
                return (args, kwargs)

>       result = Class()._function(*_args, **_kwargs)

tests/test_outer_classmethod.py:202: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_outer_classmethod.py:183: in _decorator
    self.assertEqual(instance, Class)
E   AssertionError: None != <class 'test_outer_classmethod.TestCallin[57 chars]ass'>
_____________ TestSynchronized.test_synchronized_outer_classmethod _____________

self = <test_synchronized_lock.TestSynchronized testMethod=test_synchronized_outer_classmethod>

    def test_synchronized_outer_classmethod(self):
        # Prior to Python 3.9 this isn't detected as a class method
        # call, as the classmethod decorator doesn't bind the wrapped
        # function to the class before calling and just calls it direct,
        # explicitly passing the class as first argument. For more
        # details see: https://bugs.python.org/issue19072

        if PYXY < (3, 9):
            _lock0 = getattr(C4.function2, '_synchronized_lock', None)
        else:
            _lock0 = getattr(C4, '_synchronized_lock', None)
        self.assertEqual(_lock0, None)

        c4.function2()

        if PYXY < (3, 9):
            _lock1 = getattr(C4.function2, '_synchronized_lock', None)
        else:
            _lock1 = getattr(C4, '_synchronized_lock', None)
>       self.assertNotEqual(_lock1, None)
E       AssertionError: None == None

tests/test_synchronized_lock.py:181: AssertionError
----------------------------- Captured stdout call -----------------------------
function2
=========================== short test summary info ============================
FAILED tests/test_outer_classmethod.py::TestCallingOuterClassMethod::test_class_call_function
FAILED tests/test_outer_classmethod.py::TestCallingOuterClassMethod::test_instance_call_function
FAILED tests/test_synchronized_lock.py::TestSynchronized::test_synchronized_outer_classmethod
======================== 3 failed, 435 passed in 0.83s =========================
py313: exit 1 (1.12 seconds) .../wrapt> python -m coverage run --rcfile .../wrapt/setup.cfg -m pytest -v .../wrapt/tests pid=1586485
.pkg: _exit> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
  py313: FAIL code 1 (3.27=setup[2.15]+cmd[1.12] seconds)
  evaluation failed :( (3.33 seconds)

When searching the issues here, I found https://github.com/GrahamDumpleton/wrapt/issues/160 which looks very similar, if not the same.

So I searched https://docs.python.org/3.13/whatsnew/3.13.html for "classmethod" and found:

Removed chained classmethod descriptors (introduced in gh-63272). This can no longer be used to wrap other descriptors such as property. The core design of this feature was flawed and caused a number of downstream problems. To "pass-through" a classmethod, consider using the __wrapped__ attribute that was added in Python 3.10. (Contributed by Raymond Hettinger in gh-89519.)

GrahamDumpleton / wrapt

classmethod tests fail with Python 3.13 (Python reverted to pre-3.9 behavior) #259