Formatting issues not getting caught in TravisCI

[onlinejudge95]

It has come into the observation that files that are not black compliant are still passing the Travis CI build. Due to which our master has failing checks. Can we do something about it?

Reference #2041

[cclauss]

See the second commit on #2051

The GitHub Action autoblack (not Travis CI) autoformats our code. It only runs when a maintainer (with the commit bit) creates a PR. This approach gives us eventual compliance with psf/black without having to teach every first-time contributor what psf/black is and how to use it.