...
.tox/py/lib/python2.7/site-packages/pybel/io/ndex_utils.py:19: in <module>
from ndex2.client import Ndex2 as Ndex
.tox/py/lib/python2.7/site-packages/ndex2/__init__.py:6: in <module>
from nicecxModel.metadata.MetaDataElement import MetaDataElement
.tox/py/lib/python2.7/site-packages/nicecxModel/metadata/MetaDataElement.py:3: in <module>
from nicecxModel.cx import CX_CONSTANTS
.tox/py/lib/python2.7/site-packages/nicecxModel/cx/__init__.py:3: in <module>
from enum import Enum
E ImportError: No module named enum
See https://travis-ci.org/pybel/pybel/jobs/309130831
Enum module is Python3 only and needs to be installed in setup.py for backwards compatability (https://pypi.python.org/pypi/enum34)