Remove last references to obsolete py.code

Also remove some unnecessary references to py.builtin

Fix #305

xdist/report.py

@@ -1,4 +1,4 @@
-import py
+from __future__ import print_function
 from difflib import unified_diff
 
 
@@ -17,10 +17,10 @@ def report_collection_diff(from_collection, to_collection, from_id, to_id):
         fromfile=from_id,
         tofile=to_id,
     )
-    error_message = py.builtin._totext(
-        'Different tests were collected between {from_id} and {to_id}. '
-        'The difference is:\n'
-        '{diff}'
+    error_message = (
+        u'Different tests were collected between {from_id} and {to_id}. '
+        u'The difference is:\n'
+        u'{diff}'
     ).format(from_id=from_id, to_id=to_id, diff='\n'.join(diff))
     msg = "\n".join([x.rstrip() for x in error_message.split("\n")])
     return msg