Use pyppeteer.tabs and imgmath for pdf builds

[rantahar]

Fixes equations in pdf builds and expands group tabs.

Depends on rkdarst/sphinx_pyppeteer_builder#2

[rkdarst]

For the pyppeteer part: I've been trying to avoid integrating pyppeteer into lessons too much, since it seems quite fragile and like something that needs to be replaced someday. This is conditions on the pdf tag so it can't break the rest of the build, but I'd hope we can remove this someday.

I wonder if we can directly improve tabs CSS or contribute the extension this to sphinx-tabs?

[rantahar]

Maybe. Expanding the tabs requires handling them quite differently, there would be several new if statements in sphinx-tabs code. Pure CSS could probably do it, but it would need to reorder elements, to put the titles back in sequential order when sphinx-tabs moved them to the tabs.

sphinx-tabs explicitly lists builders it supports, and the pyppeteer builder is not one of them. At minimum it would have to be included there.

[rantahar]

Moving to playwright would require a bit of work, probably implementing a small sphinx builder, but should not be that hard. The simplest way to handle tags would still be using a custom extension to replace sphinx-tabs.