Run GitHub CI tests on Python 3.13

Author: frwickst

.github/workflows/tests.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Set up python
         uses: actions/setup-python@v5
         with:
-          python-version: 3.12
+          python-version: 3.13
       #----------------------------------------------
       #  -----  install & configure poetry  -----
       #----------------------------------------------