Ensure PostgreSQL network address types are not cast as VARCHAR

Ensure PostgreSQL network address types are not cast as VARCHAR

Fixed issue in PostgreSQL network types :class:`_postgresql.INET`,
:class:`_postgresql.CIDR`, :class:`_postgresql.MACADDR`,
:class:`_postgresql.MACADDR8` where sending string values to compare to
these types would render an explicit CAST to VARCHAR, causing some SQL /
driver combinations to fail.  Pull request courtesy Denis Laxalde.

Fixes: #12060
Closes: #12412
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12412
Pull-request-sha: 029fda7f2d182af71ebc48aef191aa9114927f28

Change-Id: Id4a502ebc119775567cacddbabef2ce9715c1a9f