[1.7.x] Fixed #24007 -- Ensure apps registry's ready before unpickling models

This prevents AppRegistryNotReady errors when unpickling Django
models from an external script.
Backport of 108b8bf85 from master.

django/db/models/base.py

@@ -1459,6 +1459,8 @@ def model_unpickle(model_id, attrs, factory):
     Used to unpickle Model subclasses with deferred fields.
     """
     if isinstance(model_id, tuple):
+        if not apps.ready:
+            apps.populate(settings.INSTALLED_APPS)
         model = apps.get_model(*model_id)
     else:
         # Backwards compat - the model was cached directly in earlier versions.