BridgeDB's tests are failing due to Twisted Trial changing the way modules are called

This is happening after the upgrade to Debian 9, because Twisted was upgraded from 14.0.2 to 16.6.0. Relative imports of helper functions in the unittests directory are now broken:

[ERROR]

Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/twisted/trial/runner.py", line 602, in loadByNames
    things.append(self.findByName(name))
  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/twisted/trial/runner.py", line 405, in findByName
    return filenameToModule(name)
  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/twisted/trial/runner.py", line 96, in filenameToModule
    return _importFromFile(fn)
  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/twisted/trial/runner.py", line 119, in _importFromFile
    module = imp.load_source(moduleName, fn, fd)
  File "./test/test_translations.py", line 14, in <module>
    from .https_helpers import DummyRequest
exceptions.ValueError: Attempted relative import in non-package