'Allow deletion of a setting in an overriden settings set (#18824)'

def test_override_settings_delete(self):

    previous_i18n = settings.USE_I18N
    with self.settings(USE_I18N=False):
        del settings.USE_I18N
        self.assertRaises(AttributeError, getattr, settings, 'USE_I18N')
    self.assertEqual(settings.USE_I18N, previous_i18n)
