Python3 and flake8 fixes for G4Extensions packages

This merge request replaces usage of dict.has_key(KEY) with KEY in dict.
Additionally it adds flake8 checks for G4Extensions packages and fixes any warnings found.
Should fix ATLASSIM-4663.

Simulation/G4Extensions/Charginos/python/CharginosConfig.py

-# Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 
 from AthenaCommon import CfgMgr
 def getCharginosPhysicsTool(name="CharginosPhysicsTool", **kwargs):
     from G4AtlasApps.SimFlags import simFlags
-    from AthenaCommon.SystemOfUnits import GeV,MeV,eplus,ns
+    from AthenaCommon.SystemOfUnits import MeV,ns
     C1Mass = eval(simFlags.specialConfiguration.get_Value().get("AMSBC1Mass", None))
     N1Mass = eval(simFlags.specialConfiguration.get_Value().get("AMSBN1Mass", None))
     C1Lifetime = eval(simFlags.specialConfiguration.get_Value().get("AMSBC1Lifetime", "-1.0"))