Currently the documentation is failing to build on ReadTheDocs. It was previously set to use Python 2.x so I've changed that to 3.x (within the RTD project). However, even though I've wiped the 'latest' and 'stable' build environments, the builds are still using Sphinx 1.8.5 and are failing as a result. (In PSyclone we're using 4.x of Sphinx.) I think maybe adding a pip_requirements.txt file in the doc directory will fix this.