feat: helper to get dependency-groups

[henryiii]

Starting work on #845.

I was going to use @sirosen's dependency-groups, but it only supports 3.9+ - even the first version was 3.9+. I don't think that was a great idea for a foundational utility, but it's what it is. So we can't use it yet. And this is pretty easy, I think we can just write our own support as long as it's tested.

I've improved on the example implementation several ways: setdefault is a safer than defaultdict. I've used generators to remove the appends and extends. And I've used *args to simplify the past_groups handling.

I think we can start with a function like this, and maybe eventually come up with a nicer interface for things in pyproject.toml. But this is simple and could be used by the fancier interface later.

[sirosen]

I was going to use @sirosen's dependency-groups, but it only supports 3.9+ - even the first version was 3.9+. I don't think that was a great idea for a foundational utility, but it's what it is.

Oh, I'm happy to add 3.8 support! In fact, rather than delaying, I just did in v1.1.0 .

I'm not sure why I wasn't more conservative about the supported versions when creating the library, but I agree that was a mistake.

[henryiii]

Great, thanks! FWIW, I don't care if the latest version is 3.9+ (or 3.13+), I just need a working 3.8 version that gets pulled by 3.8. :)

[henryiii]

FYI, the only thing left is rebasing after #877 and adding docs. Edit: I've added the docs changes from #877 here and added docs, so now it doesn't depend on that one, and either can go in first.