Paddle-python

* Add list subscription users to README
* Add cancel subscription endpoint
* Remove variable type hints as they are not supported by python 3.5

**Internal**
* Fix up refund_payment tests with `pytest.mark.skip`

* Use correct 'is not None' comparison
* Add Create plan
* Better type hints
* Add list subscription users endpoint

**Internal**
* Add refund_payment and note about mocked test
* Make sure pytest-mock is installed for tox tests
* Add generate_license_mocked test
* Add mock tests along side manual_cleanup tests
* Send datetime and date to validators in normal test run
* Use datetime.now instead of uuid in tests
* Update CONTRIBUTING with manual_cleanup info

* More type hints
* Add list transactions API
* Fix bug where all falsey values were getting remove from JSON
* Add refund payment endpoint