Using a model subclass results in an empty schema

[bugeats]

It appears that the ModelSchema is somehow unable to introspect the attributes of a model if it is a subclass of another model. Subclassing in this manner works fine in pynamodb, and it is expected that it would work here as well.

Test case:

from pynamodb.attributes import UnicodeAttribute
from pynamodb.models import Model
from marshmallow_pynamodb import ModelSchema


class Base(Model):
    class Meta:
        table_name = 'test-table'
    pk = UnicodeAttribute(hash_key=True)
    alpha = UnicodeAttribute(null=True)


class Sub(Base):
    class Meta(Base.Meta):
        pass


class BaseSchema(ModelSchema):
    class Meta:
        model = Base


class SubSchema(ModelSchema):
    class Meta:
        model = Sub


sub_schema = SubSchema()
base_schema = BaseSchema()

base_dumped = base_schema.dump(Base(pk='peekay', alpha='hi mom')).data
assert base_dumped['alpha'] == 'hi mom' # <-- PASSES OK

sub_dumped = sub_schema.dump(Sub(pk='peekay', alpha='hi mom')).data
assert sub_dumped['alpha'] == 'hi mom' # <-- FAILS