replace deprecated django.utils.six with six

CHANGES.rst

@@ -1,6 +1,8 @@
 Changes
 =======
 
+- remove reference to vendored library ``django.utils.six`` in favor of ``six`` (gh-526)
+
 2.7.0 (2019-01-16)
 ------------------
 - Add support for ``using`` chained manager method and save/delete keyword argument (gh-507)

simple_history/models.py

@@ -2,6 +2,7 @@
 
 import copy
 import importlib
+import six
 import threading
 import uuid
 import warnings
@@ -15,7 +16,6 @@
 from django.db.models import Q
 from django.db.models.fields.proxy import OrderWrt
 from django.urls import reverse
-from django.utils import six
 from django.utils.encoding import python_2_unicode_compatible, smart_text
 from django.utils.text import format_lazy
 from django.utils.timezone import now