Exceptions being raised during test run

[athos-ribeiro]

The following exception keeps showing up when running the test suite. It does not make the tests fail, but should be fixed.

tests/plugins/test_tag_and_push.py ............Exception in thread Thread-2638:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/threading.py", line 804, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.7/threading.py", line 757, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 329, in _handle_workers
    time.sleep(0.1)
  File "/home/athos/.virtualenvs/atomic-reactor2/lib/python2.7/site-packages/flexmock.py", line 987, in mock_method
    raise MethodSignatureError(_format_args(name, arguments))
MethodSignatureError: sleep(0.1)

I also occurs in CI runs