Use `whenever` instead of stdlib `datetime`

[samueljsb]

https://pypi.org/project/whenever/ makes a distinction at type-checking time between naive and tz-aware datetimes and dates. This is an advantage and in some internal project we require this library and disallow access to the stdlib datetime module.

Because these types are incompatible with the stdlib, making this change would constitute a breaking change to the API. Since this project is still zero-ver, this would be acceptable, but would complicate integration with projects that already use it.