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/test_classes.py

@@ -1923,7 +1923,7 @@ def get_row_ids_in_all_tables() -> Iterator[Tuple[str, Set[int]]]:
         table_name = model._meta.db_table
         if table_name in ignored_tables:
             continue
-        ids = model.objects.all().values_list("id", flat=True)
+        ids = model._default_manager.all().values_list("id", flat=True)
         yield table_name, set(ids)