Migrate PURW_create_actual_mu_profile.py to use python 3 syntax
Fixes PURW_create_actual_mu_profile.py when run in main, which otherwise crashes due to python 3 errors.
I wasn't completely sure about replacing 'c' arrays with 'u' but I verified that running this new script (with 24.0.17) produce identical actual mu distibrutions to 21.0,latest with the following command:
PURW_create_actual_mu_profile.py --bcidfile /afs/cern.ch/atlas/www/GROUPS/DATAPREPARATION/InteractionsperCrossing/data23_13p6TeV-OflLumi-Run3-003.root --grl /cvmfs/atlas.cern.ch/repo/sw/database/GroupData/GoodRunsLists/data23_13p6TeV/20230828/data23_13p6TeV.periodAllYear_DetStatus-v110-pro31-06_MERGED_PHYS_StandardGRL_All_Good_25ns_ignoreTRIG_JETCTPIN.xml --outfile purw.2023.actualMu.root
tagging @ponyisi as the original author, and some customers in Data Prep: @hgray @cdelitzs