Make test_parser compatible with Python older than 3.8 (#575)
Make test_parser compatible with Python older than 3.8 (#575)

1. Handle importlib_metadata backport package
2. Use tuple-based unpacking of call() object

Signed-off-by: Scott K Logan <8c724d14625be4be480086a9212b212477a303ef@cottsay.net>
ros2/launch,ros2/launch,ros2/launch