Bump sphinx from 8.1.1 to 8.1.3 in /cmake/tribits/doc/sphinx (#523)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.1 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.1.1...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

cmake/tribits/doc/sphinx/requirements.txt

@@ -1,2 +1,2 @@
-Sphinx==8.1.1
+Sphinx==8.1.3
 sphinx-rtd-theme==3.0.1