Fix spurious error when asyncio.run() task raises SystemExit

[oremanj]

SystemExit passes through the asyncio loop, so the stop() callback scheduled by the main task's Future's completion doesn't run; which then means it's pending when we re-enter the asyncio loop to do cleanup tasks like shutdown_asyncgens. Fix by not enqueueing the stop() callback if we can tell that the exception is going to pass through the asyncio loop.

Relevant upstream issue: https://bugs.python.org/issue22429

[axiomiety]

looking at the exception hierarchy, do you need to include GeneratorExit here too?

[oremanj]

No; this is trying to match logic in asyncio where specifically these two exceptions will come out of the event loop if raised by a task.

asyncio/tasks.py:

class Task(...):
    ...
    def __step_run_and_handle_result(self, exc):
        coro = self._coro
        try:
            if exc is None:
                # We use the `send` method directly, because coroutines
                # don't have `__iter__` and `__next__` methods.
                result = coro.send(None)
            else:
                result = coro.throw(exc)
        ...
        except (KeyboardInterrupt, SystemExit) as exc:
            super().set_exception(exc)
            raise
        except BaseException as exc:
            super().set_exception(exc)
        else:
            ...