classmethod decorations broke in Python 3.9

[FichteFoll]

Something must have changed with how classmethods function in 3.9.

λ pytest -k test_classmethod
================================================= test session starts =================================================
platform linux -- Python 3.9.5, pytest-6.2.4, py-1.10.0, pluggy-0.13.1
rootdir: /home/fichte/code/resumeback
plugins: xonsh-0.9.27
collected 60 items / 58 deselected / 2 selected

tests/test_descriptors.py F.                                                                                    [100%]

====================================================== FAILURES =======================================================
__________________________________________________ test_classmethod ___________________________________________________

    def test_classmethod():
        ts = State()

        class B:
            @classmethod
            @send_self
            def clsmethod(this, cls, param):
                yield defer(this.next)
                assert cls is B
                ts.run = param

        b = B()
>       wait_until_finished(b.clsmethod(123))

tests/test_descriptors.py:33:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
resumeback/__init__.py:371: in __call__
    gen_wrapper.next()  # Start the first iteration
resumeback/__init__.py:109: in _next
    return self._send(generator)
resumeback/__init__.py:147: in _send
    return generator.send(value)
resumeback/__init__.py:357: in wrapper
    return (yield from self.func(gen_wrapper, *args, **kwargs))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

this = <class 'tests.test_descriptors.test_classmethod.<locals>.B'>
cls = <resumeback.GeneratorWrapper object at 0x7f68d3808430>, param = 123

    @classmethod
    @send_self
    def clsmethod(this, cls, param):
>       yield defer(this.next)
E       AttributeError: type object 'B' has no attribute 'next'

tests/test_descriptors.py:28: AttributeError
------------------------------------------------ Captured stdout call -------------------------------------------------
<bound method test_classmethod.<locals>.B.clsmethod of <class 'tests.test_descriptors.test_classmethod.<locals>.B'>> (123,)
=============================================== short test summary info ===============================================
FAILED tests/test_descriptors.py::test_classmethod - AttributeError: type object 'B' has no attribute 'next'
===================================== 1 failed, 1 passed, 58 deselected in 0.13s ======================================