Update actions/checkout action to v6 (#220)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         python-version: ["3.7", "3.8", "3.9", "3.10"]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6
         with: