Update actions/setup-python action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-python@v2
+      - uses: actions/setup-python@v6
         with:
           python-version: 3.x
       - run: pip install mkdocs