fix(attrmatch): use None to match missing attributes

[ahal]

No description provided.

[jcristau]

This feels like a slightly too subtle incompatible change. None used to only match None, now it also matches a missing attribute. That feels a bit scary, in terms of unintended consequences, plus it means there's no longer a way to match an attribute that's present but null?

[ahal]

Yes, valid points. I did consider introducing some kind of other syntax, we could set it to a constant value like missing, or we could prefix the key with ! or something like that. One complication here is that I want to be able to define the kwargs in yaml, so that precludes a sentinel object or the like.

One reason I thought this would be a good approach, is that the only place this is used (in Gecko and this repo at least), is in from-deps.with-attributes. And afaict we aren't setting anything to None there.

But we could implement something else. Do you have any preferences for handling the distinction?