Thanks everyone for the feedback.
I think the best way to handle this is to make the three next releases (3.10, 3.11, 3.12) Sphinx 2 and Sphinx 3 compatible, this would gather requiered benefits:
- Ease of backporting: from any dev version we can backport documentation changes to the previous two releases. - Ease of packaging: We're not requiering Sphinx 3 today but in like 3 years, a point where this could be discussed again to see if everyone is ready.
If this plan is OK for everyone, I'll try a PR soon™ to make this compatibility with Sphinx2/3 land in 3.10.