mypy: Upgrade mypy from 1.4.1 to 1.5.1.

_default_manager is the same as objects on most of our models. But
when a model class is stored in a variable, the type system doesn’t
know which model the variable is referring to, so it can’t know that
objects even exists (Django doesn’t add it if the user added a custom
manager of a different name). django-stubs used to incorrectly assume
it exists unconditionally, but it no longer does.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/bulk_create.py

@@ -186,7 +186,7 @@ def bulk_set_users_or_streams_recipient_fields(
         if result is not None:
             result.recipient = recipient
             objects_to_update.add(result)
-    model.objects.bulk_update(objects_to_update, ["recipient"])
+    model._default_manager.bulk_update(objects_to_update, ["recipient"])
 
 
 # This is only sed in populate_db, so doesn't really need tests