selfcheck: Enable the `redundant-expr` error code (#13547)
* selfcheck: Enable the `redundant-expr` error code
* Remove unnecessary `type: ignore`
* Revert changes to `mypy/build.py`, add a TODO
* Use a `cast` instead
* Use explicit annotation instead of `assert_type`
* Bump `types-typed-ast`, remove unneeded `type: ignore`
