Added support for Django 4.1. (#487)

* Added Django 4.0 classifier.

* Added support for Django 4.1.

constance/apps.py

@@ -1,5 +1,5 @@
 from django.db.models import signals
-from django.apps import AppConfig
+from django.apps import apps, AppConfig
 from django.utils.translation import gettext_lazy as _
 
 
@@ -18,13 +18,16 @@ class ConstanceConfig(AppConfig):
         to be able to check for permissions
         """
         from django.conf import settings
-        from django.contrib.auth.models import Permission
-        from django.contrib.contenttypes.models import ContentType
 
         constance_dbs = getattr(settings, 'CONSTANCE_DBS', None)
         if constance_dbs is not None and using not in constance_dbs:
             return
-        if ContentType._meta.installed and Permission._meta.installed:
+        if (
+            apps.is_installed('django.contrib.contenttypes') and
+            apps.is_installed('django.contrib.auth')
+        ):
+            ContentType = apps.get_model('contenttypes.ContentType')
+            Permission = apps.get_model('auth.Permission')
             content_type, created = ContentType.objects.using(using).get_or_create(
                 app_label='constance',
                 model='config',

constance/backends/database/__init__.py

@@ -21,7 +21,7 @@ class DatabaseBackend(Backend):
         self._autofill_timeout = settings.DATABASE_CACHE_AUTOFILL_TIMEOUT
         self._autofill_cachekey = 'autofilled'
 
-        if not self._model._meta.installed:
+        if self._model._meta.app_config is None:
             raise ImproperlyConfigured(
                 "The constance.backends.database app isn't installed "
                 "correctly. Make sure it's in your INSTALLED_APPS setting.")

setup.py

@@ -38,6 +38,8 @@ setup(
         'Framework :: Django :: 3.0',
         'Framework :: Django :: 3.1',
         'Framework :: Django :: 3.2',
+        'Framework :: Django :: 4.0',
+        'Framework :: Django :: 4.1',
         'Intended Audience :: Developers',
         'License :: OSI Approved :: BSD License',
         'Natural Language :: English',

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{36,37,38,39,py3}-dj{22,30,31,32}-{unittest,pytest}
-    py{38,39,310}-dj{40,main}-{unittest,pytest}
+    py{38,39,310}-dj{40,41,main}-{unittest,pytest}
     py310-dj32-{unittest,pytest}
     
 [testenv]
@@ -14,7 +14,8 @@ deps =
     dj30: Django>=3.0,<3.1
     dj31: Django>=3.1,<3.2
     dj32: Django>=3.2,<4
-    dj40: Django>=4.0a,<4.1
+    dj40: Django>=4.0,<4.1
+    dj41: Django>=4.1b1,<4.2
     djmain: https://github.com/django/django/archive/main.tar.gz
     pytest: pytest
     pytest: pytest-cov