This change adds support for Sphinx 8.0 to our pyproject.toml and

tox.ini files, and fleshes out the tox configuration a bit for wider

version compatibility checking.
This also fixes a few warnings that were popping up when using

collect_files against Sphinx 7.4:

The default for collect_file_patterns was set to a dict, but it was

  used as a list.
The "rebuild" parameter for app.add_config_value() has changed from a

  simple bool to a string defining when to rebuild. The string version

  is supported in all our supported versions.
The comparison against Sphinx's src_dir warned that we should use

pathlib.Path or os.fspath instead of a string.

Ran tests via tox.