MySQL Enum reflection convertion issue

When using reflection to build the models (instead of declarative) the Enum conversion fails because the ENUM column is treated as string so doing ```type.name``` returns None.

```sql
CREATE TABLE `foos` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`status` ENUM('open', 'closed') DEFAULT 'open',
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
```

```python
class Foo(Base):

__table__ = Table('foos', metadata, autoload=True)

```

```shell
enum_class = super(EnumMeta, metacls).__new__(metacls, cls, bases, classdict) TypeError: Error when calling the metaclass bases type() argument 1 must be string, not None
```

Suggested [fix](https://github.com/graphql-python/graphene-sqlalchemy/pull/147)