use python 3.9 for docs

Author: vincentsarago

.github/workflows/deploy_mkdocs.yml

@@ -19,10 +19,10 @@ jobs:
       - name: Checkout main
         uses: actions/checkout@v2
 
-      - name: Set up Python 3.8
+      - name: Set up Python 3.9
         uses: actions/setup-python@v2
         with:
-          python-version: 3.8
+          python-version: 3.9
 
       - name: Install dependencies
         run: |