alembic: Doesn't import custom column types like ArrowType from the right place

Migrated issue, originally created by nickretallack (@nickretallack)

I wrote a simple model in Flask-SQLAlchemy like this:

from sqlalchemy_utils.types.arrow import ArrowType
class MyModel(db.Model):
    created_at = db.Column(ArrowType)

Then I generated the migration using Flask-Migrate, which uses Alembic internally, and it generated this:

...
import sqlalchemy as sa
...
    sa.Column('created_at', sa.ArrowType(), nullable=True),
...

Of course, ArrowType is not part of sqlalchemy, so I edited it to import it from the right place. It’d be cool if it got the import correct