ensure CITEXT is not cast as VARCHAR

ensure CITEXT is not cast as VARCHAR

Fixed issue where comparisons to the :class:`_postgresql.CITEXT` datatype
would cast the right side to ``VARCHAR``, leading to the right side not
being interpreted as a ``CITEXT`` datatype, for the asyncpg, psycopg3 and
pg80000 dialects.   This led to the :class:`_postgresql.CITEXT` type being
essentially unusable for practical use; this is now fixed and the test
suite has been corrected to properly assert that expressions are rendered
correctly.

Fixes: #10096
Change-Id: I49129e50261cf09942c0c339d581ce17a26d8181