[Merge] ~renanrodrigo/update-notifier:respect-phasing into update-notifier:master

[Renan Rodrigo]

Thanks for the hint.
- You don't need the apt option to see the phased updates in `apt list --upgradable`. You will see both phased and non-phased updates there. Besides that, there are no phased updates in apt itself on Focal -, so this option will be ignored anyway. `apt show`, on the other hand, will show the package metadata which includes the phasing percentage.
I will look into improving it.

- The bug only manifests itself when phased updates are available unfortunately. We can probably fake them but I have no idea how. I am also interested on what SRUers will think...
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https://code.launchpad.net/~renanrodrigo/update-notifier/+git/update-notifier/+merge/438009
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