walk mro for Field set type annotations

[asottile]

test failed prior to this with:

_____________________ mro_is_walked_for_field_annotations ______________________
/home/asottile/workspace/django-stubs/tests/typecheck/models/test_create.yml:64: 
E   pytest_mypy_plugins.utils.TypecheckAssertionError: Output is not expected: 
E   Actual:
E     main:2: error: Incompatible type for "dct" of "Child" (got "Dict[Any, Any]", expected "Union[str, Combinable]")  [misc] (diff)
E   Expected:
E     (empty)

test is a simplified example from sentry code