build.yml: Used checkout v4

Author: schwehr

.github/workflows/build.yml

@@ -23,7 +23,7 @@ jobs:
       image: python:${{ matrix.python-version }}-slim
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip