GCS IO tests are very flaky under Python 3.5

apache_beam.io.gcp.gcsio_test.TestGCSIO.test_rename (from nosetests)

Traceback (most recent call last):
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit@2/src/sdks/python/apache_beam/utils/retry.py", line 184, in wrapper
    return fun(*args, **kwargs)
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit@2/src/sdks/python/apache_beam/io/gcp/gcsio.py", line 274, in copy
    response.totalBytesRewritten, response.objectSize, src, dest)
  File "/usr/lib/python3.5/logging/__init__.py", line 1848, in debug
    root.debug(msg, *args, **kwargs)
  File "/usr/lib/python3.5/logging/__init__.py", line 1267, in debug
    self._log(DEBUG, msg, args, **kwargs)
  File "/usr/lib/python3.5/logging/__init__.py", line 1414, in _log
    exc_info, func, extra, sinfo)
  File "/usr/lib/python3.5/logging/__init__.py", line 1384, in makeRecord
    sinfo)
  File "/usr/lib/python3.5/logging/__init__.py", line 248, in __init__
    ct = time.time()
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit@2/src/sdks/python/target/.tox/py3-gcp/lib/python3.5/site-packages/mock/mock.py", line 1062, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/home/jenkins/jenkins-slave/workspace/beam_PreCommit_Python_Commit@2/src/sdks/python/target/.tox/py3-gcp/lib/python3.5/site-packages/mock/mock.py", line 1121, in _mock_call
    result = next(effect)