Fix: salt 2018.3 + py2.7/3.x compatibility

- Fixes 'Failed to load ext_pillar reclass: Item in from list not a string'
  (https://github.com/salt-formulas/reclass/issues/27)
- Update python 2.7/3.6 compatibility
diff --git a/setup.py b/setup.py
index 5b4b8b6..789b0fd 100644
--- a/setup.py
+++ b/setup.py
@@ -6,6 +6,10 @@
 # Copyright © 2007–13 martin f. krafft <madduck@madduck.net>
 # Released under the terms of the Artistic Licence 2.0
 #
+from __future__ import absolute_import
+from __future__ import division
+from __future__ import print_function
+from __future__ import unicode_literals
 
 from reclass.version import *
 from setuptools import setup, find_packages