adding an enum db type doesn't seem to add the requisite op.drop_type() in downgrade

[lolopinto]

adding an enum e.g. rainbow_type in tests at #113

generates the following code:

# Code generated by github.com/lolopinto/ent/ent, DO NOT edit. 

"""add accounts table

Revision ID: bcfa772af8d3
Revises: 
Create Date: 2020-08-22 08:37:36.040787+00:00

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'bcfa772af8d3'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('accounts',
    sa.Column('id', sa.Integer(), nullable=False),
    sa.Column('rainbow', sa.Enum('red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet', name='rainbow_type'), nullable=False),
    sa.PrimaryKeyConstraint('id', name='accounts_id_pkey')
    )
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_table('accounts')
    # ### end Alembic commands ###

there's no explicit create type but the type is created.
however, since no explicit create type, there ends up not being a drop type either

[lolopinto]

turns out this is intentional based on how the Enum type works:

https://github.com/sqlalchemy/sqlalchemy/blob/master/lib/sqlalchemy/dialects/postgresql/base.py#L1395-L1469

When the builtin type :class:_types.Enum is used and the
:paramref:.Enum.native_enum flag is left at its default of
True, the PostgreSQL backend will use a :class:_postgresql.ENUM
type as the implementation, so the special create/drop rules
will be used.
The create/drop behavior of ENUM is necessarily intricate, due to the
awkward relationship the ENUM type has in relationship to the
parent table, in that it may be "owned" by just a single table, or
may be shared among many tables.
When using :class:_types.Enum or :class:_postgresql.ENUM
in an "inline" fashion, the CREATE TYPE and DROP TYPE is emitted
corresponding to when the :meth:_schema.Table.create and
:meth:_schema.Table.drop
methods are called::
table = Table('sometable', metadata,
Column('some_enum', ENUM('a', 'b', 'c', name='myenum'))
)
table.create(engine) # will emit CREATE ENUM and CREATE TABLE
table.drop(engine) # will emit DROP TABLE and DROP ENUM

[lolopinto]

^ didn't work. probably/maybe because of how alembic calls the create types in autogenerate mode which is different from what the comment said. So fixed in #117 to explicitly create/drop types and not depend on the table