ReadTheDocs won't build because of missing Sphinx entry in config

[RobertoRoos]

See for example: https://app.readthedocs.org/projects/sphinx-argparse/builds/30358651/

Expected behavior changed in December last year: https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/