Fix for python 3.9
This fixes the runtime failures in https://lhcb-nightlies.web.cern.ch/nightly/lhcb-lcg-dev4/1697/, which uses Python 3.9. The fix is compatible also for Python 3.8 so can be merged as normal after the standard RTA validation
This fixes the runtime failures in https://lhcb-nightlies.web.cern.ch/nightly/lhcb-lcg-dev4/1697/, which uses Python 3.9. The fix is compatible also for Python 3.8 so can be merged as normal after the standard RTA validation