Fix check for lazily evaluated page_description

_delegated_text no longer exists in Django as of ee36e101e8

This is also how Django internally checks for lazily-translated strings (search for Promise in its codebase)

wagtail/models/__init__.py

@@ -42,7 +42,7 @@ from django.utils import timezone
 from django.utils import translation as translation
 from django.utils.cache import patch_cache_control
 from django.utils.encoding import force_str
-from django.utils.functional import cached_property
+from django.utils.functional import cached_property, Promise
 from django.utils.module_loading import import_string
 from django.utils.text import capfirst, slugify
 from django.utils.translation import gettext_lazy as _
@@ -2231,7 +2231,7 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
         # make sure that page_description is actually a string rather than a model field
         if isinstance(description, str):
             return description
-        elif getattr(description, "_delegate_text", None):
+        elif isinstance(description, Promise):
             # description is a lazy object (e.g. the result of gettext_lazy())
             return str(description)
         else: