Public inspector method to load enum list

Provide opportunity to get enums list via an inspector instance public
interface.
the engine.begin() as conn is a good pattern but it's not currently what we do elsewhere (a good chunk of tests precede the "with" statement event existing).   i think here we should be able to use provide_metadata:

   @testing.provide_metadata
    def test_my_thing(self):
        mytype = pg.ENUM('x', 'y', 'z', name='myenum', metadata=self.metadata)
        mytype.create()

       <do test>

The decorator should emit a metadata.drop_all() which *should* hit the enum, I hope. see if that works.