Conda UnsatisfiableError opencv3 -> python 2.7

https://github.com/soumendra/python-machinelearning-setup/blob/228b1420f04af2b1c4d9c1298ba35da258a9055e/py36tf.yml#L10

When trying to `conda env create -f py36tf.yml` on Mac OS High Sierra, I get
```
UnsatisfiableError: The following specifications were found to be in conflict:
  - opencv3 -> python 2.7*
  - python 3.6*
```