Use sphinx[test] for extension testing?

[jdillard]

Is it advised to install sphinx[test] when testing extensions after this change 66fa790?

I was getting ModuleNotFoundError: No module named 'defusedxml' errors, but using sphinx[test] vs just sphinx in my tox deps fixed it.

I know the testing framework isn't well documented currently, but wondering if this should be added to the docs in the future if that is indeed the case.

[picnixz]

I prefer having that topic as an issue rather as a discussion, so I'm closing it. I've suggested having a sphinx[dev] and a sphinx[test] to make the distinction clearer (it might happen that Adam decides otherwise and we'll be back to a discussion but I feel that an issue is easier in this case).

See: #12339