TST: update role for caption in expected html output (sphinx 4.1)

[jorisvandenbossche]

Sphinx 4.1 added a role="heading" to the caption <p> (sphinx-doc/sphinx#9358), so this updates the expected output to get CI passing again.

In general, do we have some way to deal with such cross-sphinx-version differences? Or just test the output with the latest sphinx and ensure the expected html is up to date with that? (although ideally we would test against multiple sphinx versions?)

Closes #437

[drammock]

In general, do we have some way to deal with such cross-sphinx-version differences? [...] ideally we would test against multiple sphinx versions

This ought to be possible by specifying different sphinx versions in some of your CI recipes, and decorating some tests with pytest fixtures so they get skipped if the testing environment doesn't have the right sphinx version.

[jorisvandenbossche]

Yes, that would indeed be possible.

Now, already going to merge this PR since that makes some more builds green, as long as we are only testing for "latest sphinx". Will open another issue for multi-sphinx-version testing/support.