Python3 reproducibility

Python 3 is less happy assigning relative ordering to objects - see discussion on ATR-20541.

This fixes errors in the py3 build, e.g.

  File "/cvmfs/atlas-nightlies.cern.ch/repo/sw/master--py3/2020-02-10T2136/Athena/22.0.10/InstallArea/x86_64-centos7-gcc8-opt/python/TriggerMenuMT/HLTMenuConfig/Menu/HLTCFConfig.py", line 73, in createStepFilterNode
    stepCF = parOR(name + CFNaming.FILTER_POSTFIX, subs=sorted(set(filter_list)))
TypeError: '<' not supported between instances of 'RoRSeqFilter' and 'RoRSeqFilter'

Cheers,

Stewart