DOC: use GitHub's dependabot to manage doc/python_requirements.txt ?

[neutrinoceros]

Question

While it's good practice to pin down exact requirements for documentation builds in a requirement file (as we do now), in my experience this tends to not be future-proof as indirect dependencies release incompatible versions and break builds.
A common solution is to use GitHub's dependabot service to periodically issue update PRs so the requirements do not rot away into inevitable breakage.

This would also necessitate integrating test builds into the CI.
I'm happy to do all of that when I'm back from vacation.