TrigValTools: fix py3 issue and drop py2 compatibility

Rafal Bielskirequested to merge
rbielski/athena:tvs-py3 into master

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

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