MultipleChoiceField: align stubs with DRF 3.17 list-based API

[Copilot]

I have made things!

DRF 3.17 (encode/django-rest-framework#9735) switched MultipleChoiceField from set-based to list-based semantics, but the stubs still expected set for default/initial and inherited ChoiceField.to_internal_value returning str.

• rest_framework-stubs/fields.pyi — MultipleChoiceField:
  • default / initial now accept Sequence[...] instead of set[...].
  • Override to_internal_value to return list[Any].
  • Generic params updated: _VT → list[Any], _RP → list[str | tuple[str | int, str | int]].
• tests/typecheck/test_fields.yml — MultipleChoiceField_default updated: list literals/callables now accepted, set literals/callables flagged.

# Now valid
MultipleChoiceField(choices=[1], default=[1])
MultipleChoiceField(choices=[1], default=lambda: [1])

# Now rejected (was previously accepted)
MultipleChoiceField(choices=[1], default={1})

Related issues

• Closes MultipleChoiceField should take/return list values in DRF 3.17 #990