Upgrade setup-python action to v6

Author: fabaff

.github/workflows/testing.yml

@@ -19,7 +19,7 @@ jobs:
       uses: actions/checkout@v6
 
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}