Add 'six' to requirements. Module for 2to3 compatibility.
Add 'six' to requirements. Module for 2to3 compatibility.

plotly/python-api,plotly/plotly.py,plotly/python-api,ee-in/python-api,plotly/plotly.py,ee-in/python-api,plotly/python-api,plotly/plotly.py,ee-in/python-api