Enum ChoiceType column results in error

Howdy folks!

The following code produces `TypeError: 'MyEnum' object is not iterable`, but should work. Note that this is also involves package `sqlalchemy_utils`

```py
from enum import Enum

class MyEnum(Enum):
foo = 1
bar = 2

class MyModel(Base):
id = Column(sa.Integer, primary_key=True)
name = Column(sa.Unicode(255))
type = Column(ChoiceType(MyEnum, impl=Integer()))

class MyObject(SQLAlchemyObjectType):
class Meta:
model = MyModel
```

Start by running the supplied MyEnum, ChoiceType, and SQLAlchemyObjectType reproduction to confirm the TypeError. Trace how the SQLAlchemy integration handles the ChoiceType column, then verify that generating the object type succeeds without the enum iteration error.

Escrita pelo modelo de indexação a partir do texto da issue.