Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
         python-version: [ '3.10', '3.11', '3.12', '3.13' ]
     name: Python ${{ matrix.python-version }} tests
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Setup Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6
         with: