Counter: fix return value of most_common() (#1491)

https://docs.python.org/3/library/collections.html#collections.Counter.most_common
https://docs.python.org/2/library/collections.html#collections.Counter.most_common

stdlib/3/collections/__init__.pyi

@@ -125,7 +125,7 @@ class Counter(Dict[_T, int], Generic[_T]):
 
     def elements(self) -> Iterator[_T]: ...
 
-    def most_common(self, n: int = ...) -> List[_T]: ...
+    def most_common(self, n: Optional[int] = ...) -> List[Tuple[_T, int]]: ...
 
     @overload
     def subtract(self, __mapping: Mapping[_T, int]) -> None: ...