Mark first argument of __[get|set|del]attr__ as str (#9245)

stdlib/unittest/mock.pyi

@@ -82,7 +82,7 @@ class _Call(tuple[Any, ...]):
     def __eq__(self, other: object) -> bool: ...
     def __ne__(self, __other: object) -> bool: ...
     def __call__(self, *args: Any, **kwargs: Any) -> _Call: ...
-    def __getattr__(self, attr: Any) -> Any: ...
+    def __getattr__(self, attr: str) -> Any: ...
     def __getattribute__(self, attr: str) -> Any: ...
     if sys.version_info >= (3, 8):
         @property