Update documentation_test.yml

Be specific about python version

.github/workflows/documentation_test.yml

@@ -21,6 +21,8 @@ jobs:
         id: pages
         uses: actions/configure-pages@v2
       - uses: actions/setup-python@v3
+        with:
+          python-version: 3.9
       - name: Install dependencies
         run: |
           pip install -r requirements.txt