change default value to ... in inspect.pyi (#1998)

This is in our style guide. This is the last piece of offending code; I just submitted PyCQA/flake8-pyi#10 to enforce the rule in the linter in the future.

Author: JelleZijlstra

stdlib/3/inspect.pyi

@@ -203,7 +203,7 @@ ArgInfo = NamedTuple('ArgInfo', [('args', List[str]),
                                  ])
 
 def getargvalues(frame: FrameType) -> ArgInfo: ...
-def formatannotation(annotation: object, base_module: Optional[str] = None) -> str: ...
+def formatannotation(annotation: object, base_module: Optional[str] = ...) -> str: ...
 def formatannotationrelativeto(object: object) -> Callable[[object], str]: ...
 def formatargspec(args: List[str],
                   varargs: Optional[str] = ...,