Fix RTD buillds post Software Sources version bump

[ptheywood]

#67 increased the sphinx version to no longer be several major version behind.

This change resulted in the RTD build failing, due to the use of a deprecated and removed sphinx function, which was only called when building on RTD.

This PR:

• Adjusts the GitHub Actions workflow to run as if it were an RTD build (i.e. it defines the required environment variable to trigger the behaviour)
• Fixes the error

[ptheywood]

CI for a5d36ed failed as intended, demonstrating CI now reproduces RTD behaviour (build log)

While e37a8b2 passed, so this should now fix our RTD builds.