stack/lamp: fix using python3

We were using python2 after extending python2, this explicitly set
python:part = python3
18 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed Cloudooo.UnitTest-Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

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passed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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passed SlapOS.KVM.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SoftwareReleases.IntegrationTest-Jerome.Work

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failed SlapOS.SoftwareReleases.IntegrationTest-Master

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch

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