[Dependabot]: Update sphinx requirement

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)
- [Commits](sphinx-doc/sphinx@v5.3.0...v6.1.3)

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

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

doc/requirements.txt

@@ -3,7 +3,7 @@
 pyTooling>=4.0.1, <5.0
 
 # Enforce latest version on ReadTheDocs
-sphinx>=5.3.0, <6.0
+sphinx>=5.3.0, <7.0
 
 # Sphinx Extenstions
 #sphinx.ext.coverage