handle enum type for nested ChoiceFields

[etene]

Hello,

On a project I'm working on, I'm using Django IntegerFields, with a choices parameter, nested in postgres ArrayFields. This translates to a ListField with a child ChoiceField in the ModelSerializer.

The issue I'm having is that ChoiceFieldInspector fails to properly infer the resulting enum type, even though it could because it can guess it correctly for IntegerFields:

• First, it tries to get the corresponding model field, but since the nested ChoiceField has no source, it fails. I fixed it by using parent.source.
• Then, even if it manages to get the corresponding model field, it's an ArrayField so get_basic_type_info doesn't return anything. I fixed it by using the base_field if present.

This PR adds some code that fixes the issue & a test. Feel free to tell me if I solved the problem the wrong way; tests seem to pass so I don't think I broke anything.

[axnsan12]

Seems good, thank you!