Fixed #26013 -- Moved django.core.urlresolvers to django.urls.

Thanks to Tim Graham for the review.

tests/messages_tests/urls.py

@@ -2,10 +2,10 @@ from django import forms
 from django.conf.urls import url
 from django.contrib import messages
 from django.contrib.messages.views import SuccessMessageMixin
-from django.core.urlresolvers import reverse
 from django.http import HttpResponse, HttpResponseRedirect
 from django.template import engines
 from django.template.response import TemplateResponse
+from django.urls import reverse
 from django.views.decorators.cache import never_cache
 from django.views.generic.edit import FormView