Change `str` to `Union[str, unicode]` where it seems safe.
I didn't change it in non-covariant contexts since that can
break user code.
I manually verified a subset of the changes by calling the
relevant function with unicode literals (ascii-only).
I'm using `Union[str, unicode]` instead of just `unicode`
so that the stub still works with tools that don't promote
`str` objects to `unicode`.
* Fix stubs for 2.7/dateutil. They were pretty broken.
(The 3/dateutil share some of the brokenness but that's still a TODO.)
* Fix argparse stubs.
- Container is not strong enough for choices.
- add_subparsers() returns something with an add_parser() method.
