Backport workflow: Extend to allow labeling open PRs

[The-Compiler]

Yep, I was thinking about that as well - that would probably need to be a slightly different trigger (or even a separate workflow) though, which triggers when a PR is merged and then checks that PR's labels. I think ideally we would have both, so that the labels can be applied at any time and the proper thing will happen.

@bluetech is this something you'd be interested in? Personally I should take care of some dayjob stuff at the moment, so pytest 7 keeps me busy enough 🙂

Originally posted by @The-Compiler in #9430 (comment)

[webknjaz]

Shameless plug (yet again!): https://github.com/apps/patchback. I've accounted for applying the labels both before and after merging.