Added Lock.locked() method stub (#633)

https://docs.python.org/3/library/asyncio-sync.html#asyncio.Lock.locked

stdlib/2and3/threading.pyi

@@ -91,6 +91,7 @@ class Lock:
     else:
         def acquire(self, blocking: bool = ...) -> bool: ...
     def release(self) -> None: ...
+    def locked(self) -> bool: ...
 
 
 class _RLock: