Fix crash on Python 3.x and Django >= 1.5

mongoengine/django/sessions.py

@@ -1,7 +1,10 @@
 from django.conf import settings
 from django.contrib.sessions.backends.base import SessionBase, CreateError
 from django.core.exceptions import SuspiciousOperation
+try:
     from django.utils.encoding import force_unicode
+except ImportError:
+    from django.utils.encoding import force_text as force_unicode
 
 from mongoengine.document import Document
 from mongoengine import fields