Update checkout action to v6

Author: fabaff

.github/workflows/testing.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
     - name: Checkout
-      uses: actions/checkout@v4
+      uses: actions/checkout@v6
 
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v5