I found that cleo has typing, but mypy in my project which used cleo throw errors like this:
Cannot find implementation or library stub for module named 'cleo'

It's because cleo doesn't have py.typed marker file and not PEP-561 compatible

@sdispater If you think it is a good idea, I can create this file, include it to package and fix a few type-errors in the project in a new PR.