Adjust choices= type variance for model fields

[flaeppe]

Avoid getting stuck in an invariance pit. I don't think it makes sense to mix two tuple with named group elements in the same choices sequence(?).

This also changes the outermost container type to Sequence as e.g. both tuple and list are supported.

Reason behind this is that I'm getting the following complaints from mypy:

error: Incompatible return value type (got "List[Tuple[Any, str]]", expected "List[Union[Tuple[Any, Any], Tuple[str, Iterable[Tuple[Any, Any]]]]]")  [return-value]
note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
note: Consider using "Sequence" instead, which is covariant

Docs: https://docs.djangoproject.com/en/4.1/ref/models/fields/#choices

---

I'm not sure if we should change _FieldChoices too? e.g.

_FieldChoices = Union[Iterable[_Choice], Iterable[_ChoiceNamedGroup]]