Update sphinx-toolbox requirement from ~=3.8.3 to ~=3.9.0

Updates the requirements on [sphinx-toolbox](https://github.com/sphinx-toolbox/sphinx-toolbox) to permit the latest version.
- [Release notes](https://github.com/sphinx-toolbox/sphinx-toolbox/releases)
- [Changelog](https://github.com/sphinx-toolbox/sphinx-toolbox/blob/master/doc-source/changelog.rst)
- [Commits](sphinx-toolbox/sphinx-toolbox@v3.8.3...v3.9.0)

---
updated-dependencies:
- dependency-name: sphinx-toolbox
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -1,4 +1,4 @@
 sphinx<8.3
 furo~=2024.8.6
-sphinx-toolbox~=3.8.3
+sphinx-toolbox~=3.9.0
 sphinx-autodoc-typehints~=3.1