Bump sphinx-toolbox from 3.8.3 to 3.9.0

Bumps [sphinx-toolbox](https://github.com/sphinx-toolbox/sphinx-toolbox) from 3.8.3 to 3.9.0.
- [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
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

pyproject.toml

@@ -93,7 +93,7 @@ optional-dependencies.dev = [
     "sphinx-paramlinks==0.6",
     "sphinx-pyproject==0.3.0",
     "sphinx-substitution-extensions==2025.3.3",
-    "sphinx-toolbox==3.8.3",
+    "sphinx-toolbox==3.9.0",
     "sphinxcontrib-httpdomain==1.8.1",
     "sphinxcontrib-spelling==8.0.1",
     "sybil==9.1.0",