update to GH workflow to use actions/checkout@v4

Author: Michael-J-Ward

.github/workflows/CI.yml

@@ -12,7 +12,7 @@ jobs:
         os: ["ubuntu"]
     runs-on: ${{ matrix.os }}-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - uses: actions/setup-python@v2
         with:
           python-version: "3.10"