bpo-34872: Fix self-cancellation in C implementation of asyncio.Task

bpo-34872: Fix self-cancellation in C implementation of asyncio.Task

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.