RTD no longer automatically generates config files.
The `.readthedocs.yaml` is now required: https://blog.readthedocs.com/migrate-configuration-v2/
The `conf.py` for Sphinx is also required: https://blog.readthedocs.com/doctool-without-configuration-file/
- The previously automatically generated `conf.py` contained
`sphinx_rtd_theme` -> in order to keep it we have to set it and add it
as a requirement.
- The version field in `conf.py.in` remains uncofigured but I don't
think it is a problem because it doesn't shows up in the html docs.
Aleš Matěj