Finalize python2/3 codebase compatibility and get rid of 2to3

tests/fields/test_url_field.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 import pytest
 
+from builtins import str
+
 from mongoengine import *
 
 from tests.utils import MongoDBTestCase
@@ -35,9 +37,8 @@ class TestURLField(MongoDBTestCase):
         with pytest.raises(ValidationError) as exc_info:
             link.validate()
         assert (
-            unicode(exc_info.value)
-            == u"ValidationError (Link:None) (Invalid URL: http://\u043f\u0440\u0438\u0432\u0435\u0442.com: ['url'])"
-        )
+            str(exc_info.exception)
+            == u"ValidationError (Link:None) (Invalid URL: http://\u043f\u0440\u0438\u0432\u0435\u0442.com: ['url'])")
 
     def test_url_scheme_validation(self):
         """Ensure that URLFields validate urls with specific schemes properly.