`ensure_future` converts the input as future if it's not already a future.
The condition is the following:

if futures.isfuture(coro_or_future):
    ...
elif coroutines.iscoroutine(coro_or_future):
    ...
elif inspect.isawaitable(coro_or_future):
    ...

In real world, `ensure_future` is mostly called by `run_until_complete` and gather with async function call (coroutine) as input.
We should check `iscoroutine` first to make it most efficient.