diff --git a/model_utils/tests/fields.py b/model_utils/tests/fields.py
index 0c95815..3f1503a 100644
--- a/model_utils/tests/fields.py
+++ b/model_utils/tests/fields.py
@@ -1,13 +1,5 @@
 from django.db import models
-
-try:
-    unicode()
-    str_class = basestring
-except NameError:
-    str_class = str
-
-def with_metaclass(meta, base=object):
-    return meta("NewBase", (base,), {})
+from django.utils.six import with_metaclass, string_types
 
 
 class MutableField(with_metaclass(models.SubfieldBase, models.TextField)):
@@ -17,7 +9,7 @@ class MutableField(with_metaclass(models.SubfieldBase, models.TextField)):
             return None
 
         try:
-            if isinstance(value, str_class):
+            if isinstance(value, string_types):
                 return [int(i) for i in value.split(',')]
         except ValueError:
             pass
diff --git a/model_utils/tracker.py b/model_utils/tracker.py
index dad46f2..77eaaa4 100644
--- a/model_utils/tracker.py
+++ b/model_utils/tracker.py
@@ -28,7 +28,7 @@ class FieldInstanceTracker(object):
             self.saved_data[field] = deepcopy(field_value)
 
     def current(self, fields=None):
-        """Return dict of current values for all tracked fields"""
+        """Returns dict of current values for all tracked fields"""
         if fields is None:
             fields = self.fields
         return dict((f, self.get_field_value(f)) for f in fields)
@@ -41,7 +41,7 @@ class FieldInstanceTracker(object):
             raise FieldError('field "%s" not tracked' % field)
 
     def previous(self, field):
-        """Return currently saved value of given field"""
+        """Returns currently saved value of given field"""
         return self.saved_data.get(field)
 
     def changed(self):
@@ -61,7 +61,7 @@ class FieldTracker(object):
         self.fields = fields
 
     def get_field_map(self, cls):
-        """Return dict mapping fields names to model attribute names"""
+        """Returns dict mapping fields names to model attribute names"""
         field_map = dict((field, field) for field in self.fields)
         all_fields = dict((f.name, f.attname) for f in cls._meta.local_fields)
         field_map.update(**dict((k, v) for (k, v) in all_fields.items()