Support argument groups in add_argument_to typing

[KapJI]

add_argument_to() calls add_argument() on the supplied object and then returns that same object. Argument groups support this operation at runtime, but the current ArgumentParser annotation rejects them.

Type the parameter using a TypeVar bounded by argparse's shared _ActionsContainer base. The bound permits parsers and argument groups, while the TypeVar preserves the concrete return type: passing an ArgumentParser returns anArgumentParser, and passing an _ArgumentGroup returns an _ArgumentGroup.

Add an annotated regression test covering an argument group.

[casperdcl]

Thanks!