Add queue.Queue.empty() stub (#1259)

Fixes #1242. Seems like one method was omitted by accident.

stdlib/3/queue.pyi

@@ -11,6 +11,7 @@ class Full(Exception): ...
 
 class Queue(Generic[_T]):
     def __init__(self, maxsize: int = ...) -> None: ...
+    def empty(self) -> bool: ...
     def full(self) -> bool: ...
     def get(self, block: bool = ..., timeout: Optional[float] = ...) -> _T: ...
     def get_nowait(self) -> _T: ...