Update sphinx requirement from <8.2.0 to <9.1.0

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/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v9.0.1)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.0.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements.txt

@@ -10,7 +10,7 @@ pytest-cov>=2.6.0
 
 flake8>=3.5.0
 
-sphinx<8.2.0
+sphinx<9.1.0
 sphinx_book_theme
 sphinxcontrib-bibtex
 sphinx-math-dollar