Field for Postgres RangeField not work on Django 2.2.x( at least)

I'm using Django 2.2.x, graphene-django 2.3.0

I use a Postgres DateTimeRangeField, but GraphQL reports error ""User Error: expected iterable, but did not find one for field XXX"

I digged in, and found in `graphql`: `execution/executor.py`
```
586 assert isinstance(result, Iterable), (
587 "User Error: expected iterable, but did not find one " + "for field {}.{}."
588 ).format(info.parent_type, info.field_name)
```

where result is NonNull, inside is DateTimeTZRange. which is not declared List(DateTime) declared inside graphene-django converter.py
```
233 @convert_django_field.register(RangeField)
234 def convert_posgres_range_to_string(field, registry=None):
235 inner_type = convert_django_field(field.base_field)
236 if not isinstance(inner_type, (List, NonNull)):
237 inner_type = type(inner_type)
238 return List(inner_type, description=field.help_text, required=not field.null)
```

I want to define a data type to show structure for Range like:
```
{
lower
upper
bounds
}
```
which will introduce break changes to current API.

is it OK to do this. or what else better solution to this?