Fix Python 3 compatibility

In Python 3, dict.keys doesn't return a list, but a set-like object that represents a view of the dictionary's keys and doesn't support indexing. Formula therefor didn't worked with Python 3.

grains/init.sls

@@ -40,7 +40,7 @@ class GrainMaker:
 
   def _parseGrainValues(self, grain):
     # Parses individual grains and their values passed from the pillar
-    grainKey = grain.keys()[0]
+    grainKey = next(iter(grain.keys()))
     if grain[grainKey]:
       for value in grain[grainKey]:
         self._addToGrains(grain, grainKey, value)