`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

Lisez Lib/functools.py autour de la ligne 979, en vous concentrant sur la manière dont singledispatchmethod.get encapsule la méthode liée. Exécutez la reproduction fournie avec inspect.signature et sig.bind ; c’est terminé lorsque la signature liée omet self, accepte l’argument réel de la méthode et que les tests concernés réussissent.