ci: downgrade sphinx to v3.x
Sphinx 4.0 was recently released, but sadly the theme we're using is not yet compatible with Sphinx 4.0.
So let's limit the sphinx-version we're using to a version before Sphinx 4.0 for now.
Closes: #4843 (closed)
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
Sphinx 4.0 was recently released, but sadly the theme we're using is not yet compatible with Sphinx 4.0.
So let's limit the sphinx-version we're using to a version before Sphinx 4.0 for now.
Closes: #4843 (closed)