Add tests to avoid future regressions #13374
Description
Is your feature request related to a problem? Please describe.
I, like many others, hit #13352, and would prefer if this sort of thing didn't happen in the future -- it's very inconvenient when the CI pipeline starts failing due to a broken dependency.
Describe the solution you'd like
Please include in Sphinx's testing suite regression tests to ensure that Sphinx updates don't break popular third party extensions. Or, if there does need to be a breaking change, give the maintainers of the other packages sufficient warning to provide a fix before the new release.
Describe alternatives you've considered
Um ... mkdocs? 😆