Code review comment for lp://staging/~evfool/update-manager/fix665173

I have tested it manually on a source not supporting changelogs, and the message is only shown once, as expected. Do you mean a unit-test? I think I could do that with a hard-coded launchpad changelog URL, that reports that changelogs are not supported. I'll check, I haven't written any Python unit-tests so fat, but I assume that the right place for this would be another method in the tests/test_changelog.py