`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

阅读 Lib/functools.py 第 979 行附近的内容，重点关注 singledispatchmethod.get 如何包装绑定方法。运行提供的 inspect.signature 和 sig.bind 复现；当绑定签名省略 self、接受方法的实际参数，并且相关测试通过时，即表示完成。