argparse: type subparser helper's add_parser

[DMRobertson]

A drive-by. I grepped for TODO and this one seemed easy enough.

[DMRobertson]

Oh. Stubtest isn't happy because the actual method takes **kwargs. I don't think there's a way to express "this function uses the kwargs of another one" in this case, even with ParamSpec?

[Akuli]

You're right: there's no good way to say "same parameters as this other method" apart from aliasing the methods to each other, as in alias = original_method in the class body.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[Akuli]

Thanks! I like how there's a comment in both places that must stay in sync :)