Bump sphinx from 7.4.2 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.4.2 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@v7.4.2...v8.1.3)

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

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

website/requirements.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright (c) Contributors to the OpenEXR Project.
 
-sphinx == 7.4.2
+sphinx == 8.1.3
 breathe
 sphinx-press-theme