Some ReadTheDocs builds are failing

[UniversalSuperBox]

Right now, our Catalan build is failing: https://readthedocs.org/projects/docsubportscom-catalan/builds/12108936/

But our English build is not: https://readthedocs.org/projects/docsubportscom/builds/12108281/

I've traced this back to the use of --use-feature 2020-resolver in our English build but not Catalan. This feature was recently enabled by ReadTheDocs on a random set of projects. readthedocs/readthedocs.org#7412

This leads to Sphinx 3.2.1 being installed on the English build while 1.8.5, the ReadTheDocs attempted forced version, on Catalan.

I'd prefer having the newer Sphinx since Rediraffe doesn't support Sphinx under 2.0. Specifying the newer Sphinx in our requirements.txt should do it, since ReadTheDocs allows it to override its own default packages.

[UniversalSuperBox]

Fixed by #356