Python3 mock doesn't have assert_called_once

Oops. With python 2.x we use pypi's mock module but with python 3.x it's built
in (under unittest.mock). I thought Python bundled an exact copy of the
upstream module but seems it doesn't include the assert_called_once method...

  ======================================================================
  ERROR: test_event_listing_with_malformed_event
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/usr/lib/python3.5/unittest/mock.py", line 1157, in patched
      return func(*args, **keywargs)
    File "/home/atagar/Desktop/stem/test/unit/control/controller.py", line 688, in test_event_listing_with_malformed_event
      self.malformed_listener.assert_called_once()
    File "/usr/lib/python3.5/unittest/mock.py", line 583, in __getattr__
      raise AttributeError(name)
  AttributeError: assert_called_once