Add __future__.generator_stop for Python 3.5

[graingert]

No description provided.

[gvanrossum]

I think it would be better to add these to stdlib/3/future.py using something like

import sys
if sys.version_info[:2] >= (3, 5):
    generator_stop = ...  # type: _Feature

(despite python/mypy#698).

[graingert]

@gvanrossum yeah I think so to

[gvanrossum]

Thanks!