Make Lock usable as contect manager (#1314)

stdlib/3/multiprocessing/__init__.pyi

@@ -9,6 +9,8 @@ from multiprocessing.process import current_process as current_process
 class Lock():
     def acquire(self, block: bool = ..., timeout: int = ...) -> None: ...
     def release(self) -> None: ...
+    def __enter__(self) -> 'Lock': ...
+    def __exit__(self, exc_type, exc_value, tb) -> None: ...
 
 class AsyncResult():
     def get(self, timeout: float = ...) -> Any: ...