Mark inspect.getmembers as Optional for both Py2 and Py3 (#2172)

stdlib/3/inspect.pyi

@@ -39,7 +39,7 @@ if sys.version_info < (3, 6):
     def getmoduleinfo(path: str) -> Optional[ModuleInfo]: ...
 
 def getmembers(object: object,
-               predicate: Callable[[Any], bool] = ...,
+               predicate: Optional[Callable[[Any], bool]] = ...,
                ) -> List[Tuple[str, Any]]: ...
 def getmodulename(path: str) -> Optional[str]: ...