TYP: contextmanager expects a Generator
`contextmanager` should expect a `Generator`, not an `Iterator`. `Iterator[...]` and `Generator[..., None, None]` are not the same, see https://github.com/python/typeshed/issues/2772

This might help @asottile a bit with https://github.com/python/typeshed/pull/7430. Basically, staled because it would impact too many projects - after this PR there is one fewer :)
Thanks @twoertwein (would be cool if autotyping could have a rule for this :) )