Use "..." for attribute values, instead of None, [], {}

stdlib/3/collections/__init__.pyi

@@ -35,7 +35,7 @@ class MutableString(UserString, MutableSequence): ...
 # Technically, deque only derives from MutableSequence in 3.5.
 # But in practice it's not worth losing sleep over.
 class deque(MutableSequence[_T], Generic[_T]):
-    maxlen = 0 # type: Optional[int] # TODO readonly
+    maxlen = ... # type: Optional[int] # TODO readonly
     def __init__(self, iterable: Iterable[_T] = ...,
                  maxlen: int = ...) -> None: ...
     def append(self, x: _T) -> None: ...