Allow overridden _SubParsersAction
Allow overridden _SubParsersAction

Use an `issubclass` check rather than `==` to allow subclassing of `argparse._SubParsersAction` in applications.

Allows useful extensions such as aliasing of subcommands:
https://gist.github.com/sampsyo/471779
kislyuk/argcomplete,kislyuk/argcomplete