fix: add notfound() View method; auto-append trailing slash

the latter provides auto-redirect to `/widgets/` when user visits
`/widgets` for example

tests/views/test_base.py

@@ -3,7 +3,7 @@
 from unittest import TestCase
 
 from pyramid import testing
-from pyramid.httpexceptions import HTTPFound, HTTPForbidden
+from pyramid.httpexceptions import HTTPFound, HTTPForbidden, HTTPNotFound
 
 from wuttjamaican.conf import WuttaConfig
 from wuttaweb.views import base
@@ -31,6 +31,10 @@ class TestView(TestCase):
         form = self.view.make_form()
         self.assertIsInstance(form, Form)
 
+    def test_notfound(self):
+        error = self.view.notfound()
+        self.assertIsInstance(error, HTTPNotFound)
+
     def test_redirect(self):
         error = self.view.redirect('/')
         self.assertIsInstance(error, HTTPFound)