Update actions/checkout action to v6 (#21)

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

Author: renovate[bot]

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         os: [macos-10.15, macos-11, macos-12]
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
       - name: Install dependencies
         run: |
           python3 --version