attempt to fix inconistent use ob tabs and spaces

One of the python3 unit tests (http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/nicos_web_areamasterpy3_Athena_x86_64centos7gcc8opt64BC7G8AthenaOpt/NICOS_TestLog_2019-11-14T2131/Control_AthenaConfiguration___AthenaConfigurationConf__AthenaConfigurationTest__m.html) fails early with: TabError: inconsistent use of tabs and spaces in indentation

This change is an attempt to fix this problem.