mypy on GitHub Actions is ignoring `setup.cfg`

When `mypy` is running in the GitHub workflow [`.github/workflows/check.yml`](https://github.com/itu-algorithms/itu.algs4/blob/master/.github/workflows/check.yml), it seems to be ignoring that [`setup.cfg`](https://github.com/itu-algorithms/itu.algs4/blob/master/setup.cfg) specifies the directory `itu/` not to be checked. What is the reason? Fix this.