Fixed #22688 -- Documented ready() may be called more than once

        separate from the production settings, ``manage.py test`` would still

        execute some queries against your **production** database!

    .. note::

        In the usual initialization process, the ``ready`` method is only called

        once by Django. But in some corner cases, particularly in tests which

        are fiddling with installed applications, ``ready`` might be called more

        than once. In that case, either write idempotents methods, or put a flag

        on your ``AppConfig`` classes to prevent re-running code which should

        be executed exactly one time.

.. _namespace package:

Namespace packages as apps (Python 3.3+)