Printipy

- ShippingCost `express` is now optional - fixes 20
- Incrementing Jinja2 to 3.1.3

- Updating angle from float to int

Fully settled on the Python organization and layout.

Documentation included!

Tests passing with Python 3.9+

- Publish now has default values for everything
- Added `channel` to ProductExternal

Releasing Pythonic way of communicating to Printify API. Utilizing dataclasses and dataclass-json to handle data marshalling and some validations.