Nuheat

* Makes `requests` version less strict

- Move to Python 3
- Allow a temporary temperature holds to be set for a given duration by allowing a timestamp to be passed to `set_target_*()` functions

This release allows for more `Thermostat` mode control.

This release exposes more `Thermostat` properties and also adds some simple handling of expiring API sessions

Put a few fixes in place for Python 3

Releasing initial bit of functionality.