Update sphinx requirement from ~=7.4.7 to ~=8.1.3 in /fdmlpy (#44)

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@v7.4.7...v8.1.3)

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

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

fdmlpy/requirements.txt

@@ -1,3 +1,3 @@
 build~=1.2.2
-sphinx~=7.4.7
+sphinx~=8.1.3
 sphinx_rtd_theme~=3.0.1