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/2/__builtin__.pyi

@@ -653,7 +653,6 @@ class set(MutableSet[_T], Generic[_T]):
     def __lt__(self, s: AbstractSet[object]) -> bool: ...
     def __ge__(self, s: AbstractSet[object]) -> bool: ...
     def __gt__(self, s: AbstractSet[object]) -> bool: ...
-    # TODO more set operations
 
 class frozenset(AbstractSet[_T], Generic[_T]):
     @overload