bpo-30048: asyncio: fix Task.cancel() was ignored. (GH-1097)

bpo-30048: asyncio: fix Task.cancel() was ignored. (GH-1097)

when there are no more `await` or `yield (from)` before return in coroutine,
cancel was ignored.

example:

    async def coro():
        asyncio.Task.current_task().cancel()
        return 42
    ...
    res = await coro()  # should raise CancelledError