Incompatible with Sphinx 9.1: missing theme.toml

[jensens]

Sphinx 9.1.0 changed theme discovery to require theme.toml instead of theme.conf. This causes:

sphinx.errors.ThemeError: no theme named 'plone_sphinx_theme' found (missing theme.toml?)

See plone/meta#352

[stevepiercy]

plone-sphinx-theme depends on pydata-sphinx-theme. See https://pydata-sphinx-theme.readthedocs.io/en/stable/community/practices/versions.html#supported-sphinx-versions and https://github.com/pydata/pydata-sphinx-theme/blob/main/pyproject.toml#L25.