gh-127750: Restore inspect and pydoc support of singledispatchmethod (GH-130309)

The code is still flawed, because it does not recognize class and static
methods, and the first argument is not removed from the signature of
bound methods, but at least it does not worse than in 3.13 and older.

Lib/functools.py

@@ -1075,6 +1075,10 @@ class _singledispatchmethod_get:
             raise AttributeError
         return getattr(self._unbound.func, name)
 
+    @property
+    def __wrapped__(self):
+        return self._unbound.func
+
     @property
     def register(self):
         return self._unbound.register