py3: traceback.pyi: added format_stack (#125)

stdlib/3/traceback.pyi

@@ -13,5 +13,6 @@ def format_exc(limit: int = ..., chain: bool = ...) -> str: ...
 def extract_stack(f=..., limit=...): ...
 def extract_tb(traceback, limit=...): ...
 def format_list(list): ...
+def format_stack(f=..., limit=...) -> List[str]: ...
 
 # TODO add more