Add failback for stacktraces from exceptions

[zachgoldstein]

Right now we don't pick up stacktraces from bare exceptions. I think we should make an effort to do this, even though there is no guarantee that sys.exc_info() will pick up anything. In python 3.5+ we have exception.__traceback__, so we should also use this if it's available. This will also simplify some of the work in integrations.

[zachgoldstein]

Part of https://github.com/airbrake/airbrake-python/pull/73 was a change that would check for sys.exc_info() when a bare exception is used.