requirements.txt was pinning sphinx at a very old version. This version
doesn't work in recent versions of Python; the new RTD configuration
made RTD respect our requirements file, breaking the build.
Signed-off-by: Quentin Young <qlyoung@qlyoung.net>
As of Sep 25 2023, RTD projects require config files to build. This
patch is necessary for docs to continue to build.
Signed-off-by: Quentin Young <qlyoung@qlyoung.net>
