Fix an issue where StrawberryResolver.is_async was returning False for a
function decorated with asgiref's @sync_to_async.
The root cause is that in python >= 3.12 coroutine functions are market using
inspect.markcoroutinefunction, which should be checked with
inspect.iscoroutinefunction instead of asyncio.iscoroutinefunction
Releases contributed by @shmoon-kr via #3599