Recommend to use mypy error codes if applicable (#6305)

tests/mypy_test.py

@@ -207,6 +207,7 @@ def get_mypy_flags(args, major: int, minor: int, temp_name: str, *, custom_types
             "--no-implicit-optional",
             "--disallow-any-generics",
             "--warn-incomplete-stub",
+            "--show-error-codes",
             "--no-error-summary",
         ]
     if custom_typeshed: