setup: Use 2to3 when invoked by python3.

setup.py

@@ -19,6 +19,7 @@
 from setuptools import setup, find_packages
 import glob
 import os
+import sys
 import versioneer
 
 long_description = open("README.rst").read() + "\n"
@@ -67,4 +68,5 @@ setup(
     ]
   },
   test_suite='caucase.test',
+  use_2to3=sys.version_info >= (3, ),
 )