Bump sphinx from 8.1.0 to 8.1.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 8.1.0 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@v8.1.0...v8.1.3)

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

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

pyproject.toml

@@ -31,7 +31,7 @@ Funding = "https://opencollective.com/marshmallow"
 [project.optional-dependencies]
 docs = [
   "marshmallow-sqlalchemy>=0.19.0",
-  "Sphinx==8.1.0",
+  "Sphinx==8.1.3",
   "sphinx-issues==4.1.0",
 ]
 tests = ["flask-marshmallow[sqlalchemy]", "pytest"]