Bump sphinx

Bumps the python group in /smithy-docgen-core/src/main/resources/software/amazon/smithy/docgen/core/integrations/sphinx with 1 update: [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: python
...

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

smithy-docgen-core/src/main/resources/software/amazon/smithy/docgen/core/integrations/sphinx/requirements-base.txt

@@ -1,5 +1,5 @@
 # These are base requirements needed for any sphinx project output.
-Sphinx==8.0.2
+Sphinx==8.1.3
 sphinx_inline_tabs==2023.4.21
 sphinx-copybutton==0.5.2
 Pygments==2.18.0