python3 compatibility & coffea

[IzaakWN]

Since coffea is very similar to nanoAOD-tools, it would be nice to have this option in the future to speed up processing of nanoAOD files.

I have looked at it, but the main difficulty at the moment is that in CMSSW I have not found a neat way to use python3 with ROOT. Maybe it's first necessary to divorce TauFW from CMSSW (see issue #5), and/or some tricks are needed to set the ROOT version with for example

source /cvmfs/sft.cern.ch/lcg/views/LCG_96python3/x86_64-centos7-gcc8-opt/setup.sh

In any case, the code like pico.py and common help functions should be made python3 compatible in preparation by using

from __future__ import print_function

and fixing all print statements without parentheses, as well as replacing iteritems() with items(), etc.

[IzaakWN]

Having python3 would also allow for easy use of correctionlib, which will become CMS standard soon:
https://github.com/cms-nanoAOD/correctionlib

[IzaakWN]

• This PR implementing correctionlib for muon SFs will force user to CMSSW 12 or newer: Implement correctionlib JSONs for muon SFs #71
• Note lxplus will phase out CC7/SLC7 (end of June 2024), which means CMSSW 12 and older won't work on the EL9 anymore. Singularity might be a solution, but then submitting to HTCondor might become difficult, see HTCondor configuration & singularity for SLC7/CentOS7 compatibility #66