chore(deps-dev): update sphinx requirement

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.3.7...v9.1.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.1.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

pyproject.toml

@@ -28,7 +28,7 @@ dependencies = [
 dev = [
   "build>=1.2.2,<1.4.0",
   "ruff>=0.12.2,<0.15.0",
-  "sphinx>=7.3.7,<8.2.0",
+  "sphinx>=7.3.7,<9.2.0",
   "scikit-learn>=1.5,<1.8",
   "pre-commit>=4.0.1,<4.6.0"
 ]
@@ -45,7 +45,7 @@ homepage = "https://github.com/CyberAgentAILab/python-dte-adjustment"
 dev-dependencies = [
   "build>=1.2.2,<1.4.0",
   "ruff>=0.12.2,<0.15.0",
-  "sphinx>=7.3.7,<8.2.0",
+  "sphinx>=7.3.7,<9.2.0",
   "scikit-learn>=1.5,<1.8",
   "pre-commit>=4.0.1,<4.6.0"
 ]