Bump sphinx from 8.0.2 to 8.1.3 in the actions group across 1 directory (zarr-developers#2360)

Bumps the actions group with 1 update in the / directory: [sphinx](https://github.com/sphinx-doc/sphinx).


Updates `sphinx` from 8.0.2 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@v8.0.2...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: actions
...

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

Author: dependabot[bot]

pyproject.toml

@@ -78,7 +78,7 @@ gpu = [
     "cupy-cuda12x",
 ]
 docs = [
-    'sphinx==8.0.2',
+    'sphinx==8.1.3',
     'sphinx-autobuild>=2021.3.14',
     'sphinx-autoapi==3.3.2',
     'sphinx_design',