Moved IgnorePendingDeprecationWarningsMixin in django.test.utils.

This mixin is useful whenever deprecating a large part of Django.
Fixed #20444 -- Cookie-based sessions does not include a remote code execution-warning
Fixed #13285: populate_xheaders breaks caching
Fixed #20142 -- Added error handling for fixture setup

TestCase._fixture_setup disables transactions so,
in case of an error, cleanup is needed to re-enable
transactions, otherwise following TransactionTestCase
would fail.
Merge pull request #1122 from ambv/issue13285

Fixed #13285: populate_xheaders breaks caching
Fixed #20430 - Enable iterable of iterables for model choices

Allows for any iterable, not just lists or tuples, to be used as
the inner item for a list of choices in a model.
Fixed #20446 -- Documentation for SmallIntegerField does not clarify 'small'
Fixed #20335 - Documented the {% language %} template tag.

Thanks bmispelon for the suggestion and djangsters for the patch.
Merge pull request #1128 from erikr/improve-smallint-docs

Fixed #20446 -- Documentation for SmallIntegerField does not clarify 'small'
Merge branch 'allow-any-iterable-for-choices'
Fixed #14894 -- Ensure that activating a translation doesn't run into threading issues.

Thanks to maxbublis for the report and sergeykolosov for the patch.
Fixed #15201: Marked CACHE_MIDDLEWARE_ANONYMOUS_ONLY as deprecated
Merge branch 'master' of github.com:django/django
Merge pull request #1127 from filias/20142

Fixed #20142 -- Added error handling for fixture setup
Fixes a Python 3.x regression introduced in a19e9d80
Merge pull request #1135 from ambv/model_validation_fix

Fixes a Python 3.x regression introduced in a19e9d80
Fixed #20284 - Added a note about DEBUG in static file docs.

Thanks wim@ for the suggestion and Nicolas Noé for the patch.
Fixed #13958 -- problem reporting exception from \r-line-ended file

Thanks petrvanblokland for reporting and saz for the patch
Add missing imports and models to the examples in the the model layer documentation
Add missing imports and models to the examples in the view layer documentation