ci job: pin python version to 3.9

There are breaking changes with 3.10 which is failing the ci job. details in #129

.github/workflows/gh-deploy.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v2
         with:
-          python-version: '3.x'
+          python-version: '3.9'
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip