Prefer python3 over python2 for running relval monitoring

[iarspider]

This should hopefully fix errors like

Starting python2 /data/cmsbld/jenkins/workspace/ib-run-relvals/cms-bot/monitor_workflow.py timeout --signal SIGTERM 9000  cmsRun -j JobReport2.xml  step2_L1REPACK_HLT.py
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site.py", line 62, in <module>
    import os
  File "/usr/lib64/python2.7/os.py", line 49, in <module>
    import posixpath as path
  File "/usr/lib64/python2.7/posixpath.py", line 17, in <module>
    import warnings
  File "/usr/lib64/python2.7/warnings.py", line 8, in <module>
    import types
ImportError: No module named types

[cmsbuild]

A new Pull Request was created by @iarspider for branch master.

@aandvalenzuela, @smuzaffar, @cmsbuild, @iarspider can you please review it and eventually sign? Thanks.
@antoniovilela, @sextonkennedy, @rappoccio you are the release manager for this.
cms-bot commands are listed here

[smuzaffar]

@iarspider , for which arch/cmssw release cycle this happened?

[iarspider]

@smuzaffar 13_2_X for slc7: log

It also happened before, e.g. https://cmssdt.cern.ch/SDT/cgi-bin/logreader/slc7_amd64_gcc12/CMSSW_13_3_X_2023-10-25-2300/pyRelValMatrixLogs/run/136.9_RunDoubleMuon2016C/step2_RunDoubleMuon2016C.log#/ (the log is gone, but I found the link)

[smuzaffar]

@iarspider , although this change is harmless but I do not think that it will fix the issue. I do not think that issue is with monitor script itself otherwise all relvals should have failed.

This change needs to be tested for ib-run-relvals jobs for various release cycle (atleast for 5.3.X, 7.1.X, 10.6.X and 14.0.X)