Fixed #21877 -- Renamed django.apps.base to config.

django/apps/__init__.py

@@ -1,2 +1,2 @@
-from .base import AppConfig     # NOQA
+from .config import AppConfig   # NOQA
 from .registry import apps      # NOQA

django/apps/registry.py

@@ -8,7 +8,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.utils import lru_cache
 from django.utils._os import upath
 
-from .base import AppConfig
+from .config import AppConfig
 
 
 class Apps(object):

django/db/models/base.py

@@ -6,7 +6,7 @@ from functools import update_wrapper
 import warnings
 
 from django.apps import apps
-from django.apps.base import MODELS_MODULE_NAME
+from django.apps.config import MODELS_MODULE_NAME
 import django.db.models.manager  # NOQA: Imported to register signal handler.
 from django.conf import settings
 from django.core import checks