Update Units to subclass from Pint Quantity
Update Units to subclass from Pint Quantity

Summary:
- Extended Units from the Pint Quantity base class
- Replaced convert_to_ul and convert_to_nl with native pints conversion
- Fixed existing tests
- Updated tox to test for Python 3.5 (latest version)

Test Plan: Passes all unit tests

Reviewers: Vanessa

Subscribers: cojofra, jim

Differential Revision: https://work.r23s.net/D4157

merrywhether/autoprotocol-python