[backport] Update generateGeometry script to python3

[srimanob]

PR description:

Backport of #34875

[cmsbuild]

A new Pull Request was created by @srimanob (Phat Srimanobhas) for CMSSW_12_0_X.

It involves the following packages:

• Configuration/Geometry (geometry, upgrade)

@civanch, @Dr15Jones, @makortel, @cvuosalo, @ianna, @mdhildreth, @cmsbuild, @AdrianoDee, @srimanob can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @kpedro88, @fabiocos, @slomeo, @vargasa this is something you requested to watch as well.
@perrotta, @dpiparo, @qliphy you are the release manager for this.

cms-bot commands are listed here

[srimanob]

@cmsbuild please test

[srimanob]

@cmsbuild please abort

[srimanob]

@cmsbuild please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-406667/18342/summary.html
COMMIT: dc6aaef
CMSSW: CMSSW_12_0_X_2021-09-06-1100/slc7_amd64_gcc900
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmssw/35166/18342/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: 39
• DQMHistoTests: Total histograms compared: 2998564
• DQMHistoTests: Total failures: 0
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2998542
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 38 files compared)
• Checked 165 log files, 37 edm output root files, 39 DQM output files
• TriggerResults: no differences found

[srimanob]

+Upgrade

[cvuosalo]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_12_0_X IBs (tests are also fine) and once validation in the development release cycle CMSSW_12_1_X is complete. This pull request will now be reviewed by the release team before it's merged. @perrotta, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)

[perrotta]

+1