fix for py3 issue in AtlasSemantics.py

from past.builtins import basestring to fix warning in py3 build

Control/AthenaConfiguration/python/AtlasSemantics.py

 # Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 
+from past.builtins import basestring
 import GaudiConfig2.semantics
 from GaudiKernel.GaudiHandles import PrivateToolHandleArray
 import re