Update django.urls imports

polymorphic/admin/childadmin.py

@@ -4,7 +4,7 @@ The child admin displays the change/delete view of the subclass model.
 import inspect
 
 from django.contrib import admin
-from django.core.urlresolvers import resolve
+from django.urls import resolve
 from django.utils import six
 from django.utils.translation import ugettext_lazy as _
 

polymorphic/admin/parentadmin.py

@@ -10,9 +10,9 @@ from django.contrib.admin.helpers import AdminErrorList, AdminForm
 from django.contrib.admin.templatetags.admin_urls import add_preserved_filters
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import PermissionDenied
-from django.core.urlresolvers import RegexURLResolver
 from django.http import Http404, HttpResponseRedirect
 from django.template.response import TemplateResponse
+from django.urls import RegexURLResolver
 from django.utils.encoding import force_text
 from django.utils.http import urlencode
 from django.utils.safestring import mark_safe