Bump Python version used in tests

Author: tillprochaska

.github/workflows/backend.yml

@@ -41,7 +41,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.12"
+          python-version: "3.13"
           cache: "poetry"
           cache-dependency-path: "./backend/poetry.lock"