fix pg ENUM issues

fix pg ENUM issues

Restored the :paramref:`_postgresql.ENUM.name` parameter as optional in the
signature for :class:`_postgresql.ENUM`, as this is chosen automatically
from a given pep-435 ``Enum`` type.

Fixed issue where the comparison for :class:`_postgresql.ENUM` against a
plain string would cast that right-hand side type as VARCHAR, which due to
more explicit casting added to dialects such as asyncpg would produce a
PostgreSQL type mismatch error.

Fixes: #9611
Fixes: #9621
Change-Id: If095544cd1a52016ad2e7cfa2d70c919a94e79c1