Remove transitive imports of django.forms.utils.flattatt (#3397)

wagtail/contrib/modeladmin/options.py

@@ -4,7 +4,7 @@ from django.conf.urls import url
 from django.contrib.auth.models import Permission
 from django.core.exceptions import ImproperlyConfigured
 from django.db.models import Model
-from django.forms.widgets import flatatt
+from django.forms.utils import flatatt
 from django.utils.safestring import mark_safe
 from django.utils.translation import ugettext_lazy as _
 

wagtail/wagtailimages/models.py

@@ -11,7 +11,7 @@ from django.core import checks
 from django.core.files import File
 from django.core.urlresolvers import reverse
 from django.db import models
-from django.forms.widgets import flatatt
+from django.forms.utils import flatatt
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.functional import cached_property
 from django.utils.safestring import mark_safe