Fixes #21407: Enable I (isort) and stabilize import ordering (#21458)

- Adopt Ruff `I` (isort) rules for consistent import sorting
- Add two `# isort: split` boundaries to keep required imports pinned
  in `__init__.py` modules

netbox/utilities/testing/api.py

@@ -20,6 +20,7 @@ from netbox.models.features import ChangeLoggingMixin
 from users.constants import TOKEN_PREFIX
 from users.models import ObjectPermission, Token, User
 from utilities.api import get_graphql_type_for_model
+
 from .base import ModelTestCase
 from .utils import disable_logging, disable_warnings, get_random_string