prepare for latest pydata and python releases

[choldgraf]

I'm using this PR to make some changes to prep for the latest pydata theme and sphinx release. there's a backlog of stuff we needed to get through and some special casing we had to do, particularly around the sidebar interactivity. I'll try to close out some bugs while I'm at it, since this theme doesn't get much attention right now.

• closes Sidebar toggle buttons missing click handlers on desktop when navbar content is present #917
• closes Content boxes in dark mode #743
• closes Unreadable elements in dark mode #868
• closes Toggle primary sidebar button pops out that sidebar instead of collapsing it #865
• closes sphinx-book-theme 1.1.4 requires pydata-sphinx-theme 0.15.4 (0.16.1) #912

[choldgraf]

Things to improve

• tests are failing for a few versions
• we should simplify the test matrix to just the last three pythons and three sphinx versions
• the search, color mode, and fullscreen buttons need the blur toggle - maybe that should just be a single loop that has a list of selectors and adds the blur toggle to each so it is explicit.
• the spacing on the sidebars could be a bit nicer, so there is a bit more comfortable spacing between the edge of the sidebar and the items in it on all sides
• we should add a GitHub admonition note to the readme noting that this project is in maintenance mode, though if maintainers step up and show interest in keeping it going, we are happy to keep it going.
• We might need to bump the versions of myst nb in order to work with the latest sphinx