[3.6] bpo-30048: asyncio: fix Task.cancel() was ignored.

[methane]

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

(cherry picked from commit 991adca)
(original pull request was: GH-1097)