use python version 3.9

Author: sds100

.github/workflows/build-mkdocs.yml

@@ -17,7 +17,11 @@ jobs:
           fetch-depth: 0
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.10
+          python-version: 3.9
+      - uses: actions/cache@v3
+        with:
+          key: ${{ github.ref }}
+          path: .cache
       - run: pip install -r requirements.txt
       - run: mkdocs gh-deploy --force