Fix mypy issues found in CI/CD

It'd be a good first issue for someone to rectify the issues found in the CI/CD by `mypy`, it's non-critical, but it would definitely give a good introduction to the codebase and be a nice to have. Then that stage could also be turned on, as we currently ignore the error, from then on, we could make sure all the code is mypy compliant in the future, that'd be super useful in the long term.