The purpose of this construct is to test if the django.contrib.sites
application is installed. But in Django 1.9 it will be forbidden to
import the Site model when the django.contrib.sites application isn't
installed.
No model besides Site used this pattern.
Refs #21719, #21923.
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.
Used the information from the app cache instead of creating a duplicate
based on INSTALLED_APPS.
Model._meta.installed is no longer writable. It was a rather sketchy way
to alter private internals anyway.
When caching was activated, test_simple_sitemap would fail
because the test result was fetched from cache.
Thanks lucho for the initial patch and krzysiumed@gmail.com for
the review.
Aymeric Augustin
Russell Keith-Magee
Claude Paroz