Use codecov Github action v2

Author: takluyver

.github/workflows/main.yml

@@ -51,7 +51,7 @@ jobs:
     - name: Run the tests
       run: tox
     - name: Upload coverage to Codecov
-      uses: codecov/codecov-action@v1
+      uses: codecov/codecov-action@v2
       with:
           file: ./coverage.xml
           flags: unittests