Fix futher Django Warnings.

mayan/apps/common/widgets.py

@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 import os
 
 from django import forms
-from django.forms.util import flatatt
+from django.forms.utils import flatatt
 from django.utils.encoding import force_unicode, force_text
 from django.utils.html import conditional_escape, format_html
 from django.utils.safestring import mark_safe