python2 clean: fixed unit test to use python3

[smuzaffar]

Use python3 to load python3 ROOT module. Without this change this unit tests fails when py2 ROOT interface is dropped ( cms-sw/cmsdist#7106 )

[smuzaffar]

please test

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-34398/23796

• This PR adds an extra 16KB to repository

[cmsbuild]

A new Pull Request was created by @smuzaffar (Malik Shahzad Muzaffar) for master.

It involves the following packages:

DataFormats/FWLite (core)

@makortel, @smuzaffar, @Dr15Jones can you please review it and eventually sign? Thanks.
@makortel, @wddgit, @rovere this is something you requested to watch as well.
@silviodonato, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-35deed/16597/summary.html
COMMIT: dd45500
CMSSW: CMSSW_12_0_X_2021-07-07-2300/slc7_amd64_gcc900
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week0/cms-sw/cmssw/34398/16597/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 5158 differences found in the comparisons
• DQMHistoTests: Total files compared: 38
• DQMHistoTests: Total histograms compared: 2786260
• DQMHistoTests: Total failures: 18
• DQMHistoTests: Total nulls: 1
• DQMHistoTests: Total successes: 2786219
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: -0.004 KiB( 37 files compared)
• DQMHistoSizes: changed ( 312.0 ): -0.004 KiB MessageLogger/Warnings
• Checked 160 log files, 37 edm output root files, 38 DQM output files
• TriggerResults: no differences found

[smuzaffar]

+core

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[qliphy]

+1