Bump actions/checkout to v4 in CI

Author: flashcode

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
 
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v1