Fix sync execution crash with graphql-core 3.3 where execute_sync() would return a coroutine
instead of an ExecutionResult, causing RuntimeError: There is no current event loop,
because graphql-core 3.3's is_async_iterable default treats objects with __aiter__
(like Django QuerySets) as async iterables.
Now passes is_async_iterable=lambda _x: False during sync execution to prevent this.
Note: graphql-core >= 3.3.0a12 is now the minimum required version for the 3.3.x series.
Releases contributed by @bellini666 via #4267