gegps: Simplify open() fix.

The real issue with ec0c6956 was that Python earlier than 3.9 doesn't
allow parens around 'with' args, which are superfluous anyway.
There's no need for extra code to hide the encoding arg from Python 2.

TESTED:
Still "works" (expected error with no gpsd running) with Python 2.6,
2.7, and 3.2-3.12.