[Proton API Docs] Error making Sphinx docs on Fedora 29

When building the docs using Sphinx on Fedora 29 (ie on Sphinx 1.7.6), the use of math formulas in parts of the documentation cause the following error multiple times:

WARNING: using "math" markup without a Sphinx math extension active

This is not seen on CentOS 7, nor on Fedora 30 (Sphinx 1.8.5).