Ensure is_comparison passed for PG RANGE op() methods

Ensure is_comparison passed for PG RANGE op() methods

Fixed issue where the return type for the various RANGE comparison
operators would itself be the same RANGE type rather than BOOLEAN, which
would cause an undesirable result in the case that a
:class:`.TypeDecorator` that defined result-processing behavior were in
use.  Pull request courtesy Jim Bosch.

Fixes: #5476
Closes: #5477
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5477
Pull-request-sha: 925b117e0c91cdd67d9ddbd9d65f5ca3e88af91f

Change-Id: I52ab4d4362d379c8253990f9d328a40990a64520