Eliminate remaining field.rel deprecation warnings from tests

wagtail/wagtailadmin/utils.py

@@ -76,7 +76,7 @@ def get_object_usage(obj):
             # if the relation is between obj and an object that has a page as a
             # property, return the page
             for f in related_model._meta.fields:
-                if isinstance(f, ParentalKey) and issubclass(f.rel.to, Page):
+                if isinstance(f, ParentalKey) and issubclass(f.remote_field.model, Page):
                     pages |= Page.objects.filter(
                         id__in=related_model._base_manager.filter(
                             **{

wagtail/wagtailcore/models.py

@@ -1050,7 +1050,7 @@ class Page(six.with_metaclass(PageBase, AbstractPage, index.Indexed, Clusterable
                 continue
 
             # Ignore parent links (page_ptr)
-            if isinstance(field, models.OneToOneField) and field.rel.parent_link:
+            if isinstance(field, models.OneToOneField) and field.remote_field.parent_link:
                 continue
 
             specific_dict[field.name] = getattr(specific_self, field.name)

wagtail/wagtailsnippets/edit_handlers.py

@@ -20,7 +20,7 @@ class BaseSnippetChooserPanel(BaseChooserPanel):
     @classmethod
     def target_model(cls):
         if cls._target_model is None:
-            cls._target_model = cls.model._meta.get_field(cls.field_name).rel.model
+            cls._target_model = cls.model._meta.get_field(cls.field_name).remote_field.model
 
         return cls._target_model