[Merge] ~slyon/ubuntu-release-upgrader:slyon/focal-python2-fix into ubuntu-release-upgrader:ubuntu/focal

[Lukas Märdian]

I pushed a commit, which fixes the upgrade for the full apt-clone state of LP #1875523 as well.
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https://code.launchpad.net/~slyon/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/387057
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