The C implementation of asyncio.Task currently fails to perform the
cancellation cleanup correctly in the following scenario.
async def task1():
async def task2():
await task3 # task3 is never cancelled
asyncio.current_task().cancel()
await asyncio.create_task(task2())
The actuall error is a hardcoded call to `future_cancel()` instead of
calling the `cancel()` method of a future-like object.
Thanks to Vladimir Matveev for noticing the code discrepancy and to
Yury Selivanov for coming up with a pathological scenario.
Elvis Pranskevichus