`singledispatchmethod` inspected signature incorrectly includes `self`

Bug report

Bug description:

The result of singledispatchmethod.__get__ is wrapped to imitate the underlying function, instead of the result of func.__get__. This leads to an incorrect signature being reported by inspect.signature, which includes the leading self argument, thus misleading introspection tools as to the real arguments the method accepts.

import inspect
from functools import singledispatchmethod

class A:
    @singledispatchmethod
    def a(self, arg):
        pass

sig = inspect.signature(A().a)
# <Signature (self, arg)> but we expect <Signature (arg)>

sig.bind(None)  # should be OK because `A().a(None)` is OK. Instead:
# TypeError: missing a required argument: 'arg'
# because the first argument is erroneously bound to `self`.

One way to fix this might be to edit https://github.com/python/cpython/blob/6258844c27e3b5a43816e7c559089a5fe0a47123/Lib/functools.py#L979 to read instead

update_wrapper(_method, self.func.__get__(obj, cls))

CPython versions tested on:

3.10, 3.12

Operating systems tested on:

Linux, Windows

Leia Lib/functools.py por volta da linha 979, concentrando-se em como singledispatchmethod.get envolve o método vinculado. Execute a reprodução fornecida de inspect.signature e sig.bind; está concluído quando a assinatura vinculada omitir self, aceitar o argumento real do método e os testes relevantes passarem.