Update documentation.yaml

.github/workflows/documentation.yaml

@@ -8,6 +8,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v3
+        with:
+            python-version: '3.9'
       - name: Install dependencies
         run: |
           pip install -r requirements.txt && pip install -r docs/requirements.txt