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

[Lukas Märdian]

Lukas Märdian has proposed merging ~slyon/ubuntu-release-upgrader:slyon/focal-python2-fix into ubuntu-release-upgrader:ubuntu/focal.

Requested reviews:
  Brian Murray (brian-murray)
Related bugs:
  Bug #1875523 in ubuntu-release-upgrader (Ubuntu): "I can't upgrade the ubuntu to 20.04 by terminal. Everytime I do it, a message appear saying that it is not possible do the upgrade."
  https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1875523

For more details, see:
https://code.launchpad.net/~slyon/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/387057

Split the python_is_python2 quirk into a "prepare stage" (before calculating the dist-upgrade), where python-minimal is removed to avoid it producing any conflicts with packages depending on 'python' or 'python-minimal' and a "install stage" (after calculating the dist-upgrade), where the python-is-python2 package is installed to provide a clean upgrade path.

Fixes LP: #1875523
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