Add support for Sphinx 8.x.x

[willkg]

Sphinx 8 deprecated a couple of things and we see warnings now in the tests:

tests/conftest.py:2
  /home/willkg/mozilla/sphinx-js/tests/conftest.py:2: RemovedInSphinx90Warning: 'sphinx.testing.path' is deprecated. Use 'os.path' or 'pathlib' instead.
    from sphinx.testing.path import path

tests/test_incremental.py:8
  /home/willkg/mozilla/sphinx-js/tests/test_incremental.py:8: RemovedInSphinx90Warning: The alias 'sphinx.testing.util.strip_escseq' is deprecated, use 'sphinx.util.console.strip_colors' instead. Check CHANGES for Sphinx API modifications.
    from sphinx.testing.util import strip_escseq

I'm deprecating the project and don't plan to add support for Sphinx 9.x.x, so this is fine for now.