The change partly goes back to the old behavior for forwards migrations
which should reduce the amount of memory consumption (#24745). However,
by the way the current state computation is done (there is no
`state_backwards` on a migration class) this change cannot be applied to
backwards migrations. Hence rolling back migrations still requires the
precomputation and storage of the intermediate migration states.

This improvement also implies that Django does not handle mixed
migration plans anymore. Mixed plans consist of a list of migrations
where some are being applied and others are being unapplied.

Thanks Andrew Godwin, Josh Smeaton and Tim Graham for the review as well
as everybody involved on the ticket that kept me looking into the issue.

The ``item_enclosures`` hook returns a list of ``Enclosure`` objects which is
then used by the feed builder. If the feed is a RSS feed, an exception is
raised as RSS feeds don't allow multiple enclosures per feed item.

The ``item_enclosures`` hook defaults to an empty list or, if the
``item_enclosure_url`` hook is defined, to a list with a single ``Enclosure``
built from the ``item_enclosure_url``, ``item_enclosure_length``, and
``item_enclosure_mime_type`` hooks.

This doesn't work in Django 1.8 but was fixed in Django 1.9 as a
side effect of another change.

The issue was fixed on master due to the removal of ValuesQuerySet.

Partial forwardport of 7d471fe6 from stable/1.8.x

These steps are no longer needed as of pysqlite 2.7.0.
https://github.com/ghaering/pysqlite/commit/76f34850be582249d9048e6acb2993943b07d31b

Thanks Seth Gottlieb for help with the documentation and
Carl Meyer and Joshua Kehn for reviews.

Remove unnecessarily rude terminology.

This makes it much easier to diagnose a test failure when all
you have is the stack trace from an uncaught TemplateSyntaxError.

It didn't work anymore since the commit that enabled test
parallelization by default because parallel was equal to 0
at that point.

Since --parallel is documented not to work on Windows, it's better to
ignore it and run without parallelization than to crash. For example
this could simplify cross-platform test scripts.

As discussed on the ticket, this isn't a pattern we should recommend.

This reverts commit 47016d43.

Partial forwardport of 3cc5cc79 from
stable/1.8.x as the issue was already fixed in master in
a7bc00e1