Sphinx-code-tabs

- add ``tab`` directive for arbitrary (non-code) content
- add ``tabs`` directive and make ``code-tabs`` a backward-compatibility alias
of ``tabs``, to account for the new more general tab containers
- make the ``:title:`` option no longer required (wasn't enforced anyway by
sphinx)
- add grouped tabs
- make non-code tabs look better in latex output by boxing them like listings

- declare extension as safe for parallel reads (6)

- fix AssertionError if :title: option is missing (see 3)
- increase css specificity to fix big margins that have appeared due to some
CSS change in sphinx or rtd
- fix exception when building pdf documents (1, 4)
- prerender tab hidden/selected state to avoid content reflow on page (re-)load

- update css for sphinx 4

- no changes, release needed to trigger a new build

Initial version with basic functionality:

- all rendering is done by JS, no prerendering
- so far no "notebook-groups" that switch the language simultaneously