dep8 tests fail on non-intel

According to proposed migration
 https://people.canonical.com/~ubuntu-archive/proposed-migration/update_excuses.html

Software-properties tests have 'Always failed' on arch other than intel.

This failure is not specific to dep8 but rather can be seen just by running
the unit tests.

In a fresh non-intel container:
 (launched with 'lxc launch ubuntu-daily:cosmic c1' on ppc64 host)

 % apt-get update -q && apt-get build-dep -y software-properties
 % apt-get install bzr --no-install-recommends
 % bzr branch lp:software-properties
 % cd software-properties
 ## taken from debian/tests/run-tests
 % LC_ALL=C.UTF-8 xvfb-run -e "/tmp/xvfb.log" python3 -m unittest tests/test*.py

Or just run the failing test:
 % LC_ALL=C.UTF-8 xvfb-run -e "/tmp/xvfb.log" python3 -m unittest \
    tests.test_dbus.TestDBus.test_enable_enable_disable_source_code_sources

 $ LC_ALL=C.UTF-8 xvfb-run -e "/tmp/xvfb.log" python3 -m unittest tests.test_dbus.TestDBus.test_enable_enable_disable_source_code_sources

software-properties (0.96.24.36 to 0.96.26)

    Maintainer: Michael Vogt
    3 days old
    autopkgtest for crash/7.2.3+real-1: amd64: Pass, arm64: Pass, armhf: Pass, i386: Pass, ppc64el: Pass, s390x: Pass
    autopkgtest for livecd-rootfs/2.533: amd64: Pass, arm64: Pass, armhf: Pass, i386: Pass, ppc64el: Pass, s390x: Pass
    autopkgtest for software-properties/0.96.26: amd64: Pass, arm64: Always failed, armhf: Always failed, i386: Pass, ppc64el: Always failed, s390x: Always failed
    Valid candidate

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: software-properties-common 0.96.24.36
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic ppc64le
ApportVersion: 2.20.10-0ubuntu7
Architecture: ppc64el
Date: Mon Aug 6 17:20:32 2018
PackageArchitecture: all
ProcEnviron:
 TERM=screen-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
ProcLoadAvg: 0.15 0.11 0.08 1/2275 21089
ProcLocks:
 9: POSIX ADVISORY WRITE 187 00:3c:129 0 EOF
 14: FLOCK ADVISORY WRITE 193 00:3c:143 0 EOF
ProcSwaps:
 Filename Type Size Used Priority
 none virtual 8388544 8388544 0
ProcVersion: Linux version 4.15.0-29-generic (buildd@bos02-ppc64el-015) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #31-Ubuntu SMP Tue Jul 17 15:37:15 UTC 2018
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDump_list: total 0
cpu_cores: Number of cores present = 20
cpu_coreson: Number of cores online = 20
cpu_smt: SMT=8