CI: use Python 3.9 for building docs for now

.github/workflows/ci-build.yml

@@ -227,10 +227,10 @@ jobs:
         with:
           submodules: 'recursive'
 
-      - name: Set up Python 3.10
+      - name: Set up Python 3.9
         uses: actions/setup-python@v5
         with:
-          python-version: '3.10'
+          python-version: '3.9'
           cache: 'pip'
 
       - name: Install Python dependencies