H6G4Sim: Python 3 and flake8 fixes

Update to work with python 3.

Enable flake8 checking and fix warnings.

This merge request affects 1 package:

• LArCalorimeter/LArG4TB/H6G4Sim

Adding @pavol as watcher

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

CI Result SUCCESS (hash 5a10f3c6)

	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 9904]

The make warning is not related to this MR.

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

merged

mentioned in commit a65f6517

added sweep:ignore label