Improve mypy flags for regr_test.py and typecheck_typeshed.py (#9441)

- Remove redundant flags that are now enabled by default
- Add `--pretty` to `typecheck_typeshed.py`

tests/regr_test.py

@@ -166,7 +166,6 @@ def run_testcases(
         "--python-version",
         version,
         "--show-traceback",
-        "--show-error-codes",
         "--no-error-summary",
         "--platform",
         platform,