Add print_function for py2

rdf/helpers.py

@@ -1,5 +1,5 @@
 import sys
-from future.standard_library import install_aliases
+from future.standard_library import install_aliases, print_function
 install_aliases()
 
 from urllib import request, parse

rdf/tests.py

@@ -1,3 +1,4 @@
+from future.standard_library import print_function
 import operator
 import types
 import sys