Update sphinx to 9.1.0 (#2304)

Use Python 3.14 for build-docs in CI

Author: srittau

.github/workflows/build-docs.yml

@@ -17,7 +17,7 @@ jobs:
     - name: Set up Python
       uses: actions/setup-python@v6
       with:
-        python-version: 3.9
+        python-version: "3.14"
     - name: Install dependencies
       run: |
         pip install --upgrade pip

docs/requirements.txt

@@ -3,7 +3,7 @@
 # Sphinx version is pinned so that new versions that introduce new warnings
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
-sphinx==7.2.6
+sphinx==9.1.0
 
 # The theme used by the documentation is stored separately, so we need
 # to install that as well.