ci: run tests with python 3.9 (#45)

We want to keep 3.9 compatibility, this is a good way to achieve this.

.github/workflows/integration_tests.yaml

@@ -19,7 +19,7 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: "3.10"
+          python-version: "3.9"
 
       - name: Install dependencies
         run: |