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/run_tests.py b/run_tests.py
index 1506945..aeccb57 100755
--- a/run_tests.py
+++ b/run_tests.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
 
 import unittest
 tests = unittest.TestLoader().discover('reclass')