Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
         python-version: [3.7, 3.8, 3.9, '3.10']
 
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
     - name: Use Python ${{ matrix.python-version }}
       uses: actions/setup-python@v5
       with: