Add type annotations to the codebase

[edgarrmondragon]

This library still supports Python 2.7, so I'm not sure if this currently possible.

Another, less ideal, option would be to add stub files.

[ryanwersal]

It's an external package but there is a stub types package from typeshed.

[edgarrmondragon]

@ryanwersal oh that's probably why there wasn't any other issue for it in this repo. I'll use types-croniter then, thanks!