GitHub Actions: Upgrade to codecov v4

Author: cclauss

.github/workflows/tests.yml

@@ -33,11 +33,12 @@ jobs:
       - name: Run tests and generate coverage report
         run: coverage run
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
-        env:
-          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+        uses: codecov/codecov-action@v4
         with:
+          directory: ./coverage/reports/
           env_vars: OS,PYTHON,DJANGO
-          name: codecov-umbrella
           fail_ci_if_error: true
+          flags: unittests
+          name: codecov-umbrella
+          token: ${{ secrets.CODECOV_TOKEN }}
           verbose: true