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.