* Define __slots__ for object as Iterable[str] / Iterable[Union[str, unicode]]
* A string as __slots__ value is also valid and represents a single item
python/mypy#2380 showed a discrepancy between object and FunctionType in stdlib2. The first defined __doc__ to be str, the second Optional[str]. As FunctionType depends on object, this is no longer valid.
As suggested by @gvanrossum in python/mypy#2380, all __doc__ should be considered Optional.
(Final verdict was just to remove most __doc__ attributes since it's inherited from object.)
