Update sphinx requirement from <5,>=4 to >=4,<8

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@v4.0.0...v7.3.7)

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

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

docs/requirements.txt

@@ -21,7 +21,7 @@ python-dateutil>=2.8.2
 pytz>=2022.1
 semantic_version>=2.9.0
 six==1.16.0
-Sphinx>=4,<5
+Sphinx>=4,<8
 SphinxExtensions>=0.2.0
 sphinx_rtd_theme>=1.0.0
 Werkzeug>=2.1.2