fix import for Django 1.11

Author: Jann Haber

bootstrap3/components.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.forms.widgets import flatatt
+from django.forms.utils import flatatt
 from django.utils.safestring import mark_safe
 from bootstrap3.utils import render_tag, add_css_class
 

bootstrap3/utils.py

@@ -13,7 +13,7 @@
 except ImportError:
     from urllib.parse import urlparse, parse_qs, urlunparse
 
-from django.forms.widgets import flatatt
+from django.forms.utils import flatatt
 from django.template import Variable, VariableDoesNotExist
 from django.template.base import FilterExpression, kwarg_re, TemplateSyntaxError
 from django.template.loader import get_template