Removed the app_config.installed flag.

Since applications that aren't installed no longer have an application
configuration, it is now always True in practice.

Provided an abstraction to temporarily add or remove applications as
several tests messed with app_config.installed to achieve this effect.
For now this API is _-prefixed because it looks dangerous.

tests/admin_docs/tests.py

@@ -4,6 +4,7 @@ from django.conf import settings
 from django.contrib.sites.models import Site
 from django.contrib.admindocs import utils
 from django.contrib.auth.models import User
+from django.core.apps import app_cache
 from django.core.urlresolvers import reverse
 from django.test import TestCase
 from django.test.utils import override_settings
@@ -13,27 +14,18 @@ class MiscTests(TestCase):
     urls = 'admin_docs.urls'
 
     def setUp(self):
-        self._old_installed = Site._meta.app_config.installed
         User.objects.create_superuser('super', None, 'secret')
         self.client.login(username='super', password='secret')
 
-    def tearDown(self):
-        Site._meta.app_config.installed = self._old_installed
-
-    @override_settings(
-        SITE_ID=None,
-        INSTALLED_APPS=[app for app in settings.INSTALLED_APPS
-                        if app != 'django.contrib.sites'],
-    )
     def test_no_sites_framework(self):
         """
         Without the sites framework, should not access SITE_ID or Site
         objects. Deleting settings is fine here as UserSettingsHolder is used.
         """
-        Site._meta.app_config.installed = False
-        Site.objects.all().delete()
-        del settings.SITE_ID
-        self.client.get('/admindocs/views/')  # should not raise
+        with self.settings(SITE_ID=None), app_cache._without_app('django.contrib.sites'):
+            Site.objects.all().delete()
+            del settings.SITE_ID
+            self.client.get('/admindocs/views/')  # should not raise
 
 
 @override_settings(PASSWORD_HASHERS=('django.contrib.auth.hashers.SHA1PasswordHasher',))