`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

Lies Lib/functools.py um Zeile 979 herum und konzentriere dich darauf, wie singledispatchmethod.get die gebundene Methode umschließt. Führe die bereitgestellte inspect.signature- und sig.bind-Reproduktion aus; erledigt ist es, wenn die gebundene Signatur self auslässt, das tatsächliche Argument der Methode akzeptiert und die relevanten Tests bestehen.