PhonelistPage: Complete signature of serve()

The method wagtail.core.models.Page.serve() is documented with
the signature

    serve(request, *args, **kwargs)

http://docs.wagtail.io/en/v2.7/reference/pages/model_reference.html

Apply this signature to the serve() method of PhonelistPage. This is for
documentation purpose only, as the additional arguments are not used.

mpicms/base/models.py

@@ -126,5 +126,5 @@ class Meta: # noqa
         verbose_name = _("Phonelist")
         verbose_name_plural = _("Phonelists")
 
-    def serve(self, request):
+    def serve(self, request, *args, **kwargs):
         return ContactListView.as_view()(request)