It seems the RTD docs have not been properly deploying since the drop in python2 support, see:

https://readthedocs.org/projects/python-gitlab/builds/11110459/

A quick simple fix would probably just be to change the interpreter version to python3 in the RTD admin panel, but as a bonus it would be nice to eventually switch as much config as possible into the repo (would just need to extract some info from the RTD GUI):

https://docs.readthedocs.io/en/stable/config-file/v2.html