Choice iterator breakage against Django pre-5.0

[carltongibson]

Refs django/django#16943
Refs wagtail/wagtail#10846

  File "/Users/carlton/Projects/Django/django-filter/.tox/py311-latest/lib/python3.11/site-packages/django/forms/fields.py", line 874, in __init__
    self.choices = choices
    ^^^^^^^^^^^^
  File "/Users/carlton/Projects/Django/django-filter/django_filters/fields.py", line 264, in _set_choices
    super()._set_choices(value)
    ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'super' object has no attribute '_set_choices'

[carltongibson]

The initial ChoiceIterator changes are here:

• Fix null filtering for *Choice filters #680