Upgrade codecov GitHub action

Author: timobrembeck

.github/workflows/tests.yml

@@ -33,7 +33,7 @@ jobs:
       - name: Run tests and generate coverage report
         run: coverage run
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v1
+        uses: codecov/codecov-action@v3
         with:
           env_vars: OS,PYTHON,DJANGO
           name: codecov-umbrella