Normalized cut error on Py 36

[stefanv]

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FAIL: Test to catch an error thrown when subgraph has all equal edges.
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Traceback (most recent call last):
  File "/Users/travis/build/scikit-image/scikit-image-wheels/venv/lib/python3.6/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/Users/travis/build/scikit-image/scikit-image-wheels/venv/lib/python3.6/site-packages/numpy/testing/decorators.py", line 147, in skipper_func
    return f(*args, **kwargs)
  File "/Users/travis/build/scikit-image/scikit-image-wheels/venv/lib/python3.6/site-packages/skimage/future/graph/tests/test_rag.py", line 180, in test_ncut_stable_subgraph
    assert new_labels.max() == 0
AssertionError

https://travis-ci.org/scikit-image/scikit-image-wheels/jobs/203631060

[stefanv]

Closing as duplicate of #2450