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

This merge request affects 1 package:

• Trigger/TrigValidation/TrigValTools

Adding @okumura ,@rbielski ,@jpanduro as watchers

added Trigger master review-pending-level-1 labels

CI Result SUCCESS (hash c6cae88e)

	Athena	AthSimulation	AthGeneration	AnalysisBase
externals
cmake
make
required tests
optional tests

Full details available on this CI monitor view
Athena: number of compilation errors 0, warnings 1
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 20621]

WARNing is not related, approving.

Xiaozhong (L1)

added review-approved label and removed review-pending-level-1 label

mentioned in commit b300c197

merged

added sweep:ignore label