LArG4Validation: Python 3 / flake8 fixes

Update to work with python 3.

Also add flake8 checking and fix warnings.

This merge request affects 1 package:

• LArCalorimeter/LArG4/LArG4Validation

Adding @pavol as watcher

added JetEtmiss LAr Simulation master review-pending-level-1 labels

CI Result SUCCESS (hash b9bae94e)

	Athena	AthSimulation
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
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 9903]

The make warning is not related to this MR.

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

merged

mentioned in commit 24ec9f13

added sweep:ignore label