readthedocs build fails

Build that passed for 2.0.0: https://readthedocs.org/projects/kea/builds/14856809/

More recent build that failed for 2.0.0: https://readthedocs.org/projects/kea/builds/15107301/

2.0.0 is the same for both so obviously nothing changed on our part between these two builds, so this must be something changed on the readthedocs part.

The error seems to be inside a sphinx module. This is not very useful so I tried random fixes and I found that upgrading from sphinx 1.8.5 to 4.2.0 makes it build succesfully. This requires at least a requirements.txt file, but could be complemented by a .readthedocs.yaml file.