TrigValTools: fix py3 issue and drop py2 compatibility

This MR includes:

1. Changes in Trigger ART test timeout handling:
   • fix py3 issue: decode bytes from subprocess.check_output() to str
   • remove LCG_96 workaround for psutil module import
   • catch exception when generating backtrace / killing the timed-out process
2. General clean-up of py2 compatibility using six to simplify the code and reduce the number of imported modules