Plan and all of its dependencies support Python 3 so you can start managing your cron jobs in Python 3 Now. Though, Python 3 currently has very few users and a lot PyPI provided libraries do not support Python 3 yet. Unless you are aware of what you are doing, I recommend using Python 2.x until the whole ecosystem is ready.
If you want to use Plan with Python 3 you need to use Python 3.3 or higher. Older versions are not going to be supported.