slapos-node package: fixes for python3

Also adapt build.sh for bootstrapping with python3.

This is a fixup of 4b28ab13. See also !1154
13 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed Cloudooo.UnitTest-Master

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passed SlapOS.Eggs.UnitTest-Master.Python2

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passed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

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