NoneType not in types module in python3.

Change our tests to use type(None) instead which works in both python2 and python3.

common/lib/xmodule/xmodule/modulestore/mongoengine_fields.py

@@ -3,8 +3,6 @@ Custom field types for mongoengine
 """
 from __future__ import absolute_import
 
-from types import NoneType
-
 import mongoengine
 import six
 
@@ -38,7 +36,7 @@ class CourseKeyField(mongoengine.StringField):
         """
         # calling super b/c it decodes utf (and doesn't have circularity of from_python)
         course_key = super(CourseKeyField, self).to_python(course_key)
-        assert isinstance(course_key, (NoneType, six.string_types, CourseKey))
+        assert isinstance(course_key, (type(None), six.string_types, CourseKey))
         if course_key == '':
             return None
         if isinstance(course_key, six.string_types):
@@ -47,7 +45,7 @@ class CourseKeyField(mongoengine.StringField):
             return course_key
 
     def validate(self, value):
-        assert isinstance(value, (NoneType, six.string_types, CourseKey))
+        assert isinstance(value, (type(None), six.string_types, CourseKey))
         if isinstance(value, CourseKey):
             return super(CourseKeyField, self).validate(text_type(value))
         else:
@@ -74,7 +72,7 @@ class UsageKeyField(mongoengine.StringField):
         """
         Deserialize to a UsageKey instance: for now it's a location missing the run
         """
-        assert isinstance(location, (NoneType, six.string_types, UsageKey))
+        assert isinstance(location, (type(None), six.string_types, UsageKey))
         if location == '':
             return None
         if isinstance(location, six.string_types):
@@ -84,7 +82,7 @@ class UsageKeyField(mongoengine.StringField):
             return location
 
     def validate(self, value):
-        assert isinstance(value, (NoneType, six.string_types, UsageKey))
+        assert isinstance(value, (type(None), six.string_types, UsageKey))
         if isinstance(value, UsageKey):
             return super(UsageKeyField, self).validate(text_type(value))
         else: