Use modern imports (#4)

* add

* add

* Update apps.py

Author: baseplate-admin

django_ltree/apps.py

@@ -1,17 +1,11 @@
-import importlib
-
 from django.apps import AppConfig
 
 
-def register_pathfield():
-    # Register field checks, lookups and functions
-    importlib.import_module("django_ltree.checks")
-    importlib.import_module("django_ltree.lookups")
-    importlib.import_module("django_ltree.functions")
-
-
 class DjangoLtreeConfig(AppConfig):
+    default_auto_field = "django.db.models.BigAutoField"
     name = "django_ltree"
 
     def ready(self):
-        register_pathfield()
+        from . import checks as checks
+        from . import lookups as lookups
+        from . import functions as functions