bpo-39679: Add tests for classmethod/staticmethod singledispatchmethods (GH-29034)

In Python 3.8 and 3.9, stacking `@functools.singledispatchmethod` on top of
`@classmethod` or `@staticmethod` caused an exception to be raised if the
method was registered using type-annotations rather than
`@method.register(int)`. This was not caught by unit tests, however, as the
tests only tested the `@method.register(int)` way of registering additional
implementations. The bug is no longer present in Python 3.10+, but
`test_functools.py` is still lacking regression tests for these cases. This
commit adds these test cases.