Add maxsize attribute to Queue (#1465)

stdlib/3/queue.pyi

@@ -10,6 +10,7 @@ class Empty(Exception): ...
 class Full(Exception): ...
 
 class Queue(Generic[_T]):
+    maxsize = ...  # type: int
     def __init__(self, maxsize: int = ...) -> None: ...
     def empty(self) -> bool: ...
     def full(self) -> bool: ...