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
```

首先執行提供的 MyEnum、ChoiceType 和 SQLAlchemyObjectType 重現，以確認 TypeError。追蹤 SQLAlchemy 整合如何處理 ChoiceType 欄位，然後驗證物件型別的產生能夠成功完成，且不會出現 enum 迭代錯誤。

由索引模型根據 Issue 內容生成。