fix some TODOs (#1994)

- Made deque.maxlen read-only
- We don't support 3.2, so we don't care about signature changes in it
- There don't seem to be any missing set operations (I compared the dir() of this class to that of builtins.set)

stdlib/3/collections/__init__.pyi

@@ -187,7 +187,8 @@ class UserString(Sequence[str]):
 # methods did not exist).
 # But in practice it's not worth losing sleep over.
 class deque(MutableSequence[_T], Generic[_T]):
-    maxlen = ...  # type: Optional[int] # TODO readonly
+    @property
+    def maxlen(self) -> Optional[int]: ...
     def __init__(self, iterable: Iterable[_T] = ...,
                  maxlen: int = ...) -> None: ...
     def append(self, x: _T) -> None: ...