Bump actions/checkout from 5 to 6 (#20)

Author: dependabot[bot]

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
         python-version: ["3.8", "3.12"]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v6
         with: