Co-authored-by: Vladan Paunovic <vladan.paunovic.bg@gmail.com>

This is based on the Flask integration but includes background and websocket exceptions, and works with asgi.

* Set version check for v21.9 only
* Upgrade tests for v21.12 compat
* Add message to exception in tests

Co-authored-by: Neel Shah <neelshah.sa@gmail.com>

Prior to this change, the Celery integration always instruments
`task.run` and incorrectly instruments `task.__call__` (`task(...)` is
equivalent to `type(task).__call__(...)`, not `task.__call__(...)`).
After this change, we'll use the same logic as Celery to decide whether
to instrument `task.__call__` or `task.run`. That change allows abstract
tasks to catch/raise exceptions before the Sentry wrapper.