Use Python 3.12.

Python 3.12 is cached by the runners. 3.13 has to be downloaded by the runners every run again, so using 3.12 by default speeds up the execution.

ci/python-app.yml

@@ -19,10 +19,10 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Set up Python 3.13
+    - name: Set up Python 3.12
       uses: actions/setup-python@v5
       with:
-        python-version: "3.13"
+        python-version: "3.12"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip