Adopt 'enforce-labels' GitHub action from JupyterLab team

[yuvipanda]

Inspired by this comment (jupyterhub/kubespawner#793 (comment)) from @consideRatio, where adding labels to PRs is very helpful in creating changelogs. The JupyterLab team has a GH workflow
(https://github.com/jupyterlab/jupyterlab/blob/main/.github/workflows/enforce-label.yml) that ensures this happens on all PRs, and surfaces it more explicitly when it's not.

We could adopt this as a way to minimize toil during releases, where the person doing releases might have to go and relabel old PRs.

[minrk]

I like having this as a reminder for the PR merger. The only thing I don't like is seeing that red X as a PR contributor for something entirely out of my control and irrelevant to my contribution (when contributing to repos I don't have labeling permission on, that is). Maybe most folks don't get the same "You've done something wrong" visceral reaction from the red X that I do?

[sgibson91]

Agree with Min - it would be nice if, instead of failure, it added a "to-be-traiged" label, or left a "To the Reviewer, please add labels" comment. Which is something we could try to contribute to the action, under a feature flag or something.

[yuvipanda]

yeah i don't like the big red either.

+1 on moving it to a comment, perhaps one that even gets autodeleted after? I think that's how other projects do it

[minrk]

I like the affirmative triage label, since that's easiest to look for, since you can visit is:merged label:triage to quickly view all the PRs that need a label (is:merged -label:bug OR -label:enhancement... is a lot harder)

[manics]

I like the idea of using labels too, it avoids sending comment notifications to everyone.