GitHub CI: update actions/checkout to v7

Author: rrthomas

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v7
     - uses: actions/setup-python@v6
       with:
         python-version: '3.13'