adding an enum db type doesn't seem to add the requisite op.drop_type() in downgrade about ent HOT 2 CLOSED

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

from ent.

^ 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

from ent.