Add clear method to MutableSequence in Python 3 (#1765)

Author: sproshev

stdlib/3/typing.pyi

@@ -260,6 +260,7 @@ class MutableSequence(Sequence[_T], Generic[_T]):
     def __delitem__(self, i: slice) -> None: ...
     # Mixin methods
     def append(self, object: _T) -> None: ...
+    def clear(self) -> None: ...
     def extend(self, iterable: Iterable[_T]) -> None: ...
     def reverse(self) -> None: ...
     def pop(self, index: int = ...) -> _T: ...