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/users/apps.py

@@ -6,6 +6,7 @@ class UsersConfig(AppConfig):
 
     def ready(self):
         from netbox.models.features import register_models
+
         from . import signals  # noqa: F401
 
         # Register models