Update sphinx requirement from ~=7.2 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/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.2.0...v8.1.0)

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

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

requirements-dev.txt

@@ -3,5 +3,5 @@
 # SPDX-License-Identifier: MIT
 
 pre-commit~=3.6
-sphinx~=7.2
+sphinx~=8.1
 sphinx-rtd-theme~=2.0