use urlencode from django.utils.http (#760)

This works across django versions from at least version 1.7 and is
compatible across python versions.

https://docs.djangoproject.com/en/1.8/ref/utils/#django.utils.http.urlencode

django_filters/widgets.py

@@ -3,10 +3,7 @@
 
 from collections import Iterable
 from itertools import chain
-try:
-    from urllib.parse import urlencode
-except:
-    from urllib import urlencode  # noqa
+from django.utils.http import urlencode
 
 import django
 from django import forms