Type __call__ on builtins._NotImplementedType as None. (#12984)

Currently, this is intentionally incorrectly typed in order to produce a
better mypy error message. But pyright (and presumably other type checkers)
end up just treating instances of _NotImplementedType as callable.

With this change, the mypy error message gets a little worse, but other type
checkers can understand that instances of _NotImplementedType aren't
callable, which I think is an improvement.

Author: rchen152

stdlib/builtins.pyi

@@ -1284,9 +1284,7 @@ class property:
 
 @final
 class _NotImplementedType(Any):
-    # A little weird, but typing the __call__ as NotImplemented makes the error message
-    # for NotImplemented() much better
-    __call__: NotImplemented  # type: ignore[valid-type]  # pyright: ignore[reportInvalidTypeForm]
+    __call__: None
 
 NotImplemented: _NotImplementedType