fixed #23112 -- Field.get_choices tries to index an iterable

related to https://code.djangoproject.com/ticket/23112
Instead of commenting out this test, you could mark it as an expected failure:

```python
from unittest import expectedFailure

@expectedFailure("TODO: _get_FIELD_display doesn't work with Iterators")
def test_iterator_choices_and_field_display(self):
    ...
```