Update actions/checkout action to v2

Author: renovate-bot

.github/workflows/format.yml

@@ -9,7 +9,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           ref: ${{ github.head_ref }}
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
       - name: Set up Python 3.7
         uses: actions/setup-python@v1
         with: