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

[Lukas Märdian]

The release notes for Focal have been updated accordingly:

https://wiki.ubuntu.com/FocalFossa/ReleaseNotes#Python3_by_default
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https://code.launchpad.net/~slyon/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/387057
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