DataProcessing: use python3 in unit test

[smuzaffar]

This is needed in order to drop the next set of python2 based modules ( cms-sw/cmsdist#7112 ).
These are technical changes and should not break any thing as unit tests are working in PY3 IBs where python is python3.

[smuzaffar]

please test

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-34471/23924

• This PR adds an extra 12KB to repository

[cmsbuild]

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

It involves the following packages:

• Configuration/DataProcessing (operations)

@silviodonato, @qliphy, @fabiocos, @davidlange6 can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @fabiocos this is something you requested to watch as well.
@silviodonato, @dpiparo, @qliphy, @perrotta 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-baf583/16777/summary.html
COMMIT: 980ebf6
CMSSW: CMSSW_12_0_X_2021-07-13-1100/slc7_amd64_gcc900
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmssw/34471/16777/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: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 38
• DQMHistoTests: Total histograms compared: 2786302
• DQMHistoTests: Total failures: 1
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2786279
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 37 files compared)
• Checked 160 log files, 37 edm output root files, 38 DQM output files
• TriggerResults: no differences found

[qliphy]

+1

[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 be automatically merged.