Django-subscriptions

- Subscriptions in `SUSPENDED` state can now be `renewed()` which solves an issue
with payment callbacks correctly identifying a payment success.

- Introduced a boolean setting `SUBSCRIPTIONS_STUCK_RETRY`, defaulting to `False`, that will
cause stuck subscriptions to be marked as `renewal_failed` rather than `state_unknown`.

- Dropped support for Django < 2.2
- Dropped support for Python 2
- Made dependencies >= rather than ^ to support a greater range
- Changed the log messages generated by the celery tasks
- Added the pk of the subscription to the str/repr to aid debugging

- Transition methods that received a `reason` argument now accept a `description` argument,
which is stored on the state log, so that reasons aren't lost. Reason is still valid,
but will not be persisted to the state log. Migrate to using `description`, and it will
log in the state log AND the `reason` field.
`state_unknown`, `renewal_failed`, `end_subscription` are affected.

- SubscriptionState.choices is now sorted, so that the order is consistent between python versions,
and migrations are not generated by applications.

- **Breaking Change** `suspended_timeout` now triggers for subscriptions in SUSPENDED state that are
`timeout_hours` past the `subscription.end` time. It used to trigger if `subscription.last_updated`
hadn't changed for `timeout_hours`, but if `trigger_suspended` was running daily, the subscription
was constantly being updated, and `trigger_suspended_timeout` would never find a record to `end()`.