Update furo requirement

Updates the requirements on [furo](https://github.com/pradyunsg/furo) to permit the latest version.
- [Release notes](https://github.com/pradyunsg/furo/releases)
- [Changelog](https://github.com/pradyunsg/furo/blob/main/docs/changelog.md)
- [Commits](pradyunsg/furo@2020.11.19.beta18...2023.09.10)

---
updated-dependencies:
- dependency-name: furo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

doc-source/requirements.txt

@@ -1,6 +1,6 @@
 git+https://github.com/sphinx-toolbox/sphinx-toolbox-experimental.git
 extras-require>=0.5.0
-furo<=2021.6.18b36,>=2020.11.19b18
+furo>=2020.11.19b18,<=2023.9.10
 html-section>=0.3.0
 pytz>=2020.5
 seed-intersphinx-mapping>=1.2.2