BUG: fix highlighting of current version in switcher menu

[drammock]

closes #1128
(supersedes) closes #1305

#1344 ended up implementing most of #1305; this PR implements the remainder of #1305 (the new tests and some tweaks to the switcher ARIA roles). I gave co-author credit to @jpfeuffer here (really should have been done in #1344 but I failed to do it there, oops).

This PR also:

• fixes a (silent on our CIs!) bug in the a11y tests, introduced in ENH - Scalable colour system for theme #1174, where pretty_axe_results wasn't getting the new selector param passed to it. ignore this, I must have had a cache problem / wasn't comparing against current main
• fixes a (silent on our CIs!) bug in the translation test, introduced in fix: create a dedicated area for the prev-next buttons #1333, where the wrong class was used to select the previous-next area when checking for translations of "previous" and "next"

This PR doesn't do everything to fix accessibility of the version switcher; I've opened a separate issue to remind us to tackle that.

[drammock]

OK @12rambau CIs pass and this one should be ready to go. Highlighting of the active version works https://pydata-sphinx-theme--1357.org.readthedocs.build/en/1357/ and is being tested now (though note that our CIs currently don't "fail" when there are a11y test failures, but the CI log shows the switcher highlighting test passing (it's the last one in the a11y suite):

tests/test_a11y.py ..FFFFFF..FF........                                  [100%]

[trallard]

I am away from my computer to do a thorough review. But at a quick glance I noticed this PR reverts this back to a menu (see 544f6ba where it was changed to a listbox).

Also from that previous PR (and I was reminded again this week as I was doing a quick keyboard navigation audit) that the version switcher focus is hidden right now - so need to swap the tabindex=-1 to 0

[drammock]

I am away from my computer to do a thorough review. But at a quick glance I noticed this PR reverts this back to a menu (see 544f6ba where it was changed to a listbox).

Also from that previous PR (and I was reminded again this week as I was doing a quick keyboard navigation audit) that the version switcher focus is hidden right now - so need to swap the tabindex=-1 to 0

thanks for the notes @trallard, I must have been working from an outdated local copy of #1305. I've changed this PR to listbox and tab index 0 now.

[drammock]

this one is ready to go whenever someone has a moment to review/merge.

[trallard]

Thanks @drammock I finally had a chance to look into this PR.

Looks good except for a minor change needed. I tested on a separate branch w/main merged so that I could see the focus-visible when doing the tests, and all looks pretty good.

As a summary, this is how the keyboard navigation works now:

When the dropdown is collapsed

• TAB: place focus on the version selector
• Space and Enter opens the options list (it does not highlight the current list as this is handled separately as of now

With the dropdown expanded:

• TAB move through options, changing selection at the same time (to move backwards Shift + TAB)
• Escape closes the list, without applying the highlighted option
• Enter closes the list and applies the highlighted option
• Space does nothing

It would be nice to be able to use Up and Down arrow, but the current approach is usable through keyboard and voice control so we could merge as is

💡 I suggested removing the tabindex as I noticed this was selecting the whole list right after opening the option list which would require one to press Escape then start the navigation