Update the tests for Sphinx 8.1

[mitya57]

Fixes #199.

[foster999]

Which dependency has added the new class? As it's not functional for our package, it would be best to ignore this in our tests rather than add it to them - see the section above your change that does this for pygments

P.s. thanks for taking a look at this ☺️

[mitya57]

It is not a new dependency. It is caused by sphinx-doc/sphinx@eb6ff0a — on the contrary, the tests now use the built-in basic theme instead of alabaster which is a separate package.

Last time you told me that “it would be best for the tests to reflect the latest sphinx version”, so this is what I did here. If you prefer to remove this block like done for Pygments — let me know, I will redo.