`sphinx>=3.5` does not render tabs/math formulas

[humitos]

In Sphinx 3.5 a new feature that only adds the JS/CSS required to render the page only on pages where those JS/CSS are required instead of adding them to all the pages. This is a good optimization, but it breaks our extension when we want to render the content of a different page that requires these JS/CSS to render properly. This is the case of MathJax formulas and sphinx-tabs as well.

Upstream issue: sphinx-doc/sphinx#9115
Reference: sphinx-doc/sphinx#8631 (comment)

[humitos]

Upstream PR was merged and it will be available in Sphinx 4.1.0. mathjax formulas will start working again with that version of Sphinx.

sphinx-tabs extension needs to be updated to support this change: https://github.com/executablebooks/sphinx-tabs/blob/master/sphinx_tabs/tabs.py#L310-L327

[humitos]

I opened a PR for sphinx-tabs at executablebooks/sphinx-tabs#127