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

[Brian Murray]

Review: Approve

This looks good to me, thanks for working on it. I think this should also be merged into the master branch for ubuntu-release-upgrader so that we have the "PreDistUpgradeCache" quirks available in the future.
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https://code.launchpad.net/~slyon/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/387057
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