[CI] use Python 3.9 as that is what we have in Alma9

Author: glpatcern

.github/workflows/ci-tests.yml

@@ -15,10 +15,10 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
 
-    - name: Set up Python 3.12
+    - name: Set up Python 3.9
       uses: actions/setup-python@v3
       with:
-        python-version: "3.12"
+        python-version: "3.9"
 
     - name: Install dependencies
       run: |