support PG INHERITS in autogenerate

Migrated issue, originally created by Quentin Leffray

Running Alembic autogenerate do not detect inherited columns from a parent table, and flag them as "to-be-dropped".

The model is defined as follow:

class Animal(Base):
    __tablename__ = 'animal'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    type_ = Column(String)

    __mapper_args__ = {
        'polymorphic_identity': 'animal',
        'polymorphic_on': type_,
        'with_polymorphic': '*',
    }

class Unicorn(Animal):
    __tablename__ = 'unicorn'
    __table_args__ = {'postgresql_inherits': 'animal'}

    id = Column(Integer, ForeignKey('animal.id'), primary_key=True)
    color = Column(String)

    __mapper_args__ = {
        'polymorphic_identity': 'unicorn',
    }

And the migration produced by alembic --autogenerate:

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('unicorn', 'name')
    op.drop_column('unicorn', 'type_')

Start by reproducing the issue with the Animal and Unicorn models shown in the report and run Alembic autogenerate against PostgreSQL. Trace the autogenerate comparison that produces the drop_column operations; done means inherited name and type_ columns are not flagged for removal.