Update sphinx requirement from <1.9,>=1.8 to >=1.8,<8.1

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/v8.0.2/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v1.8.0...v8.0.2)

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

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

dev-requirements.txt

@@ -5,4 +5,4 @@ pytest
 pytest-cov
 
 # Only for building documentation
-Sphinx>=1.8,<1.9
+Sphinx>=1.8,<8.1