`all` acts as an alias for all formats ([docs](https://docs.readthedocs.io/en/stable/config-file/v2.html#formats)). Whilst there are only three formats right now, this would auto expand to other formats in the future, which seems desirable?
Backport of 1fe23bdd29a8f2f6802c2038702ff7a5d0e21a0d from main
This pins Sphinx version, because the default Sphinx version used by
RTD is not compatible with Python 3.8+.
This also, sets Python 3.8 for RTD builds which is compatible with all
current versions of Django.
Thanks to Mariusz Felisiak for the suggestion.
Backport of 447b6c866f0741bb68c92dc925a65fb15bfe7995 from main.