Update sphinx requirement from <6.0,>=1.3 to >=1.3,<9.0

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/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v1.3...v8.1.3)

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

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

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx>=1.3,<6.0
+sphinx>=1.3,<9.0
 sphinx-py3doc-enhanced-theme
 sphinxcontrib-plantuml
 setuptools_scm