Define __slots__ for object as Iterable[str] / Iterable[Union[str, unicode]] (#780)

* Define __slots__ for object as Iterable[str] / Iterable[Union[str, unicode]]
* A string as __slots__ value is also valid and represents a single item

stdlib/3.4/asyncio/events.pyi

@@ -13,7 +13,6 @@ AF_UNSPEC = 0     # from socket
 AI_PASSIVE = 0
 
 class Handle:
-    __slots__ = ... # type: List[str]
     _cancelled = False
     _args = ... # type: List[Any]
     def __init__(self, callback: Callable[..., Any], args: List[Any],

stdlib/3.4/asyncio/futures.pyi

@@ -15,7 +15,6 @@ from concurrent.futures import (
 class InvalidStateError(Error): ...
 
 class _TracebackLogger:
-    __slots__ = ... # type: List[str]
     exc = ...  # type: BaseException
     tb = ... # type: List[str]
     def __init__(self, exc: Any, loop: AbstractEventLoop) -> None: ...