Loosen ListFilter.choices() return type to allow custom filter templates

[alessio-b2c2]

Summary

• Change ListFilter.choices() return type from Iterator[_ListFilterChoices] to Iterator[Mapping[str, object]] since the base class imposes no structure on the returned dicts
• Add explicit choices() declarations on SimpleListFilter and FieldListFilter with the specific Iterator[_ListFilterChoices] return type, where Django actually enforces the dict shape
• Fixes spurious mypy [override] errors for ListFilter subclasses that use custom templates with different dict keys (e.g. multi-select filters yielding {"selected": bool, "value": str, "display": str} without query_string)

Why Mapping[str, object] instead of dict[str, Any]?

Neither mypy nor pyright consider TypedDict a structural subtype of dict (even dict[str, Any]), so using dict[str, ...] as the base return type prevents subclasses from narrowing to a TypedDict without triggering [override] errors.

TypedDict IS recognized as a subtype of Mapping by both type checkers, so Iterator[Mapping[str, object]] allows the base class to accept any dict-like return while letting subclasses narrow to Iterator[_ListFilterChoices].

Motivation

ListFilter is the base class and its choices() method raises NotImplementedError — it's meant to be overridden. The consuming code in Django (admin_list_filter template tag) just passes list(spec.choices(cl)) to the template context without enforcing any specific shape. Subclasses with custom templates can return dicts with entirely different keys, but the current stub forces the _ListFilterChoices TypedDict shape on all subclasses, causing spurious [override] errors.

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