Fix pytest (#2181)

Author: emmanuelle

Diff for: .circleci/create_conda_optional_env.sh

@@ -14,7 +14,7 @@ if [ ! -d $HOME/miniconda/envs/circle_optional ]; then
     ./miniconda.sh -b -p $HOME/miniconda
 
     # Create environment
-    # PYTHON_VERSION=3.6
+    # PYTHON_VERSION=2.7 or 3.5
     $HOME/miniconda/bin/conda create -n circle_optional --yes python=$PYTHON_VERSION \
 requests nbformat six retrying psutil pandas decorator pytest mock nose poppler xarray scikit-image ipython jupyter ipykernel ipywidgets