Python3 fix in AthenaCommon
Minor fix but allows a lot of unit tests to succeed, e.g. http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/GITWebArea/nightlies/master--py3/2019-12-15T2131/Athena/x86_64-centos7-gcc8-opt/Athena/UT/DataModelRunTests.log.html
Cheers,
Stewart