[Merge] ~enr0n/software-properties:deb822 into software-properties:ubuntu/master

[Julian Andres Klode]

Review: Needs Fixing

Can you bump Depends: python3-apt to `(>= 2.5.3~)`? I'll still have to release that of course, but um, this will prevent software-properties from breaking if you edit deb822 sources (because python-apt 2.5.2 doesn't accept strings for Deb822SourceEntry).
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https://code.launchpad.net/~enr0n/software-properties/+git/software-properties/+merge/437803
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