mypy

8cd6d898 - Show an error when old and new redefinion are enabled in a file (#20920)

Show an error when old and new redefinion are enabled in a file (#20920) I think we should add this to v1.20, as really not prohibiting this initially was a bug IMO. Btw @JukkaL all my semantic changes to `--allow-redefinition-new` are now in master. You can try flipping the switch internally.