Enforce labels on PRs

[blink1073]

Use the new action from maintainer-tools.

[codecov-commenter]

Codecov Report

Merging #613 (5e33ec9) into master (6c8c923) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #613      +/-   ##
==========================================
- Coverage   77.36%   77.34%   -0.02%     
==========================================
  Files         110      110              
  Lines       10252    10252              
  Branches     1259     1259              
==========================================
- Hits         7931     7929       -2     
- Misses       1926     1927       +1     
- Partials      395      396       +1     

Impacted Files	Coverage Δ
jupyter_server/tests/services/kernels/test_api.py	97.02% <0.00%> (-1.99%)	⬇️

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[blink1073]

I verified that the build fails when there is no label and passes when there is a label.

[Zsailer]

Thanks, Steve!