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
```