tests fail on python3.5

I got a ping from Barry Warsaw about a bit of our code breaking in Python 3.5:

https://launchpadlibrarian.net/210474153/buildlog_ubuntu-wily-amd64.checkbox-ng_0.18-1_BUILDING.txt.gz

The link (if you don't see it anymore) is showing this failure:

FAIL: test_help (commands.test_sru.TestSru)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/«PKGBUILDDIR»/checkbox_ng/commands/test_sru.py", line 76, in test_help
    self.assertEqual(io.combined, cleandoc(expected) + "\n")
AssertionError: "/«BUILDDIR»/checkbox-ng-0.1[1594 chars]ns\n" != "usage: checkbox sru [-h] [--check-config][1384 chars]ns\n"
- /«PKGBUILDDIR»/checkbox_ng/service.py:162: DeprecationWarning: inspect.Signature.from_function() is deprecated, use Signature.from_callable()
- sig = Signature.from_function(func)
  usage: checkbox sru [-h] [--check-config] --secure-id SECURE-ID
                      [--fallback FILE] [--destination URL] [--staging] [-n]
                      [-T TEST-PLAN-ID] [-i PATTERN] [-x PATTERN] [-w WHITELIST]

  optional arguments:
    -h, --help show this help message and exit
    --check-config run check-config before starting

  sru-specific options:
    --secure-id SECURE-ID
                          associate submission with a machine using this SECURE-
                          ID (unset)
    --fallback FILE if submission fails save the test report as FILE
                          (unset)
    --destination URL POST the test report XML to this URL (https://certific
                          ation.canonical.com/submissions/submit/)
    --staging override --destination to use the staging
                          certification website

  execution options:
    -n, --dry-run don't really run most jobs

  test selection options:
    -T TEST-PLAN-ID, --test-plan TEST-PLAN-ID
                          load the specified test plan
    -i PATTERN, --include-pattern PATTERN
                          include jobs matching the given regular expression
    -x PATTERN, --exclude-pattern PATTERN
                          exclude jobs matching the given regular expression
    -w WHITELIST, --whitelist WHITELIST
                          load whitelist containing run patterns

I think we can do some simple checks for python version or, if we can, just kill checkbox-ng's dbus support.