setup.py: add six as explicit dependency

Author: manu-chroma

setup.py

@@ -36,7 +36,8 @@
     'mistune >= 0.7.3, < 0.8',
     'typing >= 3.5.2, < 3.6',
     'CacheControl >= 0.11.7, < 0.12',
-    'lockfile >= 0.9']
+    'lockfile >= 0.9',
+    'six >= 1.8.0']
 
 # install_requires.append("avro")  # TODO: remove me once cwltool is
 # available in Debian Stable, Ubuntu 12.04 LTS