Support / test older versions of sphinx?

[jorisvandenbossche]

I don't know if we have some "official" stance about which versions of sphinx we support. But currently we are only testing the latest version of sphinx (and a pre-release version if available), cfr #441.

Ideally we would put a minimum bound on version of sphinx to support (3.x ?) and then also have a test environment with that version.

[choldgraf]

How about we test with the latest two major releases? And explicitly don't guarantee support outside of that