Switch 'u' arrays to bytearrays for root compatibility
@ponyisi noted an issue in !67305 (comment 7350007) that affected the tree created by the script to make actual mu. Root cannot handle python 'u'arrays as an alternative to 'c' arrays.
This MR replaces 'u' arrays with bytearray objects.
I verified that the tree contents are now correct (as in the old rel21/python 2 version of this script) and that produced actual mu distributions are still identical (as they were for the previous MR) again using:
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