bindings: python: fix __repr__() implementations

bindings: python: fix __repr__() implementations

The __repr__() function should - if possible - return a valid Python
expression that can be used to instantiate a new object when evaluated.

LineSettings.__repr__() is missing comas between arguments. Both Chip and
LineSettings also don't prefix the returned string with 'gpiod.'. Fix
both functions and add more test cases - including actually using the
strings returned by __repr__() to create new objects and compare their
contents.

Reported-by: Robert Thomas <rob.thomas@raspberrypi.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>