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/builtins.pyi

@@ -503,7 +503,6 @@ class bytearray(MutableSequence[int], ByteString):
     def __gt__(self, x: bytes) -> bool: ...
     def __ge__(self, x: bytes) -> bool: ...
 
-
 class memoryview(Sized, Container[bytes]):
     format = ...  # type: str
     itemsize = ...  # type: int
@@ -537,7 +536,6 @@ class memoryview(Sized, Container[bytes]):
     if sys.version_info >= (3, 5):
         def hex(self) -> str: ...
 
-
 class bool(int):
     def __init__(self, o: object = ...) -> None: ...
     @overload  # type: ignore
@@ -720,7 +718,6 @@ 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]):
     def __init__(self, iterable: Iterable[_T] = ...) -> None: ...
@@ -888,7 +885,6 @@ else:
              errors: Optional[str] = ..., newline: Optional[str] = ..., closefd: bool = ...) -> IO[Any]: ...
 
 def ord(c: Union[str, bytes, bytearray]) -> int: ...
-# TODO: in Python 3.2, print() does not support flush
 def print(*values: Any, sep: str = ..., end: str = ..., file: Optional[IO[str]] = ..., flush: bool = ...) -> None: ...
 @overload
 def pow(x: int, y: int) -> Any: ...  # The return type can be int or float, depending on y