[ci] add workflow to check PR labels

[jameslamb]

Adds a workflow to enforce that every PR has exactly 1 of the required labels.

To ensure they're all categorized in release notes correctly, matching this:

pydistcheck/.github/release-drafter.yml

Lines 4 to 14 in 127264d

	categories:
	- title: '💡 New Features'
	label: 'enhancement'
	- title: '😬 Breaking'
	label: 'breaking'
	- title: '🙈 Bug Fixes'
	label: 'fix'
	- title: '📖 Documentation'
	label: 'doc'
	- title: '🛠 Maintenance'
	label: 'maintenance'

Notes for Reviewers

docs: https://github.com/mheap/github-action-required-labels

[jameslamb]

✅ fails with 0 labels:

Error: Label error. Requires exactly 1 of: breaking, doc, enhancement, fix, maintenance. Found:

https://github.com/jameslamb/pydistcheck/actions/runs/13222997536/job/36910352073?pr=302

[jameslamb]

✅ fails with 1, incorrect label (triggered by a new commit)

Error: Label error. Requires exactly 1 of: breaking, doc, enhancement, fix, maintenance. Found: help wanted

https://github.com/jameslamb/pydistcheck/actions/runs/13223021215/job/36910400800?pr=302

[jameslamb]

✅ fails with 2, correct labels (triggered by a re-labeling)

Error: Label error. Requires exactly 1 of: breaking, doc, enhancement, fix, maintenance. Found: fix, breaking

https://github.com/jameslamb/pydistcheck/actions/runs/13223051370/job/36910463197?pr=302

And happy to say no other workflows were re-triggered 🎉

[jameslamb]

✅ succeeds with exactly 1 correct label (triggered by a re-labeling)

(no message)

https://github.com/jameslamb/pydistcheck/actions/runs/13223058542/job/36910478084?pr=302

This looks like it's working as advertised. I like it 😁