changed requirements.txt to include six=1.15.0

Author: jmsmdy

packages/python/plotly/requirements.txt

@@ -6,7 +6,7 @@
 ###################################################
 
 ## python 2 to 3 compatibility ##
-six==1.8.0
+six==1.15.0
 
 ## retrying requests ##
 tenacity>=6.2.0