Cleanup: use lower-case list and dict, add a test (#6161)

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
-      - run: ./tests/check_pep_604.py
+      - run: ./tests/check_new_syntax.py
 
   flake8:
     name: Lint with flake8