functools.singledispatchmethod: add __call__ for better results (#3899)

It doesn't actually have __call__, it does some descriptor stuff, but
this makes things work. _SingleDispatchCallable has a __call__ too,
which is what this mirrors.

Fixes #3898

stdlib/3/functools.pyi

@@ -86,6 +86,7 @@ if sys.version_info >= (3, 8):
         def register(self, cls: Any, method: None = ...) -> Callable[[Callable[..., _T]], Callable[..., _T]]: ...
         @overload
         def register(self, cls: Any, method: Callable[..., _T]) -> Callable[..., _T]: ...
+        def __call__(self, *args: Any, **kwargs: Any) -> _T: ...
 
     class cached_property(Generic[_T]):
         func: Callable[[Any], _T]