gnu: Add python2-six.

* gnu/packages/python.scm (python2-six): New variable.

gnu/packages/python.scm

@@ -507,6 +507,9 @@ Six supports every Python version since 2.5.  It is contained in only one
 Python file, so it can be easily copied into your project.")
     (license x11)))
 
+(define-public python2-six
+  (package-with-python2 python-six))
+
 (define-public python-dateutil-2
   (package
     (name "python-dateutil")