Update actions/checkout action to v3

Author: renovate-bot

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
         python-version: [3.6, 3.7, 3.8, 3.9]
     steps:
       - name: Checkout code 🛎️
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }} 🔧
         uses: actions/setup-python@v2
         with: