Update sphinx requirement from <7.5 to <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@v0.1.61611...v8.0.2)

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

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

dev-requirements.txt

@@ -2,7 +2,7 @@ requests>=2, <3
 pytest
 pytest-cov
 coverage
-sphinx<7.5
+sphinx<8.1
 sphinx_rtd_theme
 jinja2
 pytz