Add useful type hints to `Tap.add_subparsers`

[aucampia]

Currently, the type signature is just def add_subparsers(self, **kwargs) -> None: and this more or less eliminates all benefits of type hints, and then some. It would be much better if this function actually had accurate type hints.

[swansonk14]

Hi @aucampia,

Thank you for bringing up this issue! We believe this is similar to #69.

Best,
Jesse and Kyle