Update actions/checkout action to v3 (#11)

Co-authored-by: Renovate Bot <bot@renovateapp.com>

Author: renovate[bot]

.github/workflows/tests.yml

@@ -9,7 +9,7 @@ jobs:
   test-macos-1015:
     runs-on: macos-10.15
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Install dependencies
         run: |
           python3 --version