deque init() optional maxlen (#2585)

Closes #2581

stdlib/3/collections/__init__.pyi

@@ -192,7 +192,7 @@ class deque(MutableSequence[_T], Generic[_T]):
     @property
     def maxlen(self) -> Optional[int]: ...
     def __init__(self, iterable: Iterable[_T] = ...,
-                 maxlen: int = ...) -> None: ...
+                 maxlen: Optional[int] = ...) -> None: ...
     def append(self, x: _T) -> None: ...
     def appendleft(self, x: _T) -> None: ...
     def clear(self) -> None: ...