* Add _AttributeHolder and _ActionsContainer classes to argparse.
* Add Action subclasses to argparse.
* Add _UNRECOGNIZED_ARGS_ATTR, _ensure_value, _get_action_name to argparse.
* Fill in remaining _ActionsContainer attributes.
* Fill in missing argparse.ArgumentParser attributes.
* Fill in missing argparse.HelpFormatter attributes.
* Fill in remaining missing attributes on argparse classes.
* Rename TypeVar _ActionVar to _ActionT
* Add a version check for FileType attributes
* Add '# undocumented' where appropriate
* Add more # undocumented comments
* Make arguments to _ActionsContainer.add_argument more precise.
* ArgumentParser's add_argument returns the Action that was created.
* Make add_argument's dest parameter Optional. _ArgumentGroup's add_argument should also return the Action.
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.
