Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/pythonpackage.yml

@@ -11,7 +11,7 @@ jobs:
         python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v6
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v1
       with: