Update actions/checkout action to v4

Author: mend-for-github-com[bot]

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         python-version: ['3.9', '3.10', '3.11']
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Set Environment Variables
         id: set_env_vars