Ensure pyright types are still intact after using the mixin

[ddanier]

See pydantic/pydantic#857 (comment) and pydantic/pydantic#857 (comment)

@ddanier thank you for that! It works really well. The only issue is it removes the pyright types:

Without AsyncValidationModelMixin:

With AsyncValidationModelMixin:

AND

@ddanier I found a fix for the typing issue; in metaclasses.py line 17; you can change it to (make sure to add the missing imports):

@dataclass_transform(kw_only_default=True, field_specifiers=(Field,))
class AsyncValidationModelMetaclass(ModelMetaclass):