Add HTTP status code 451 to status.py

The Internet Engineering Task Force approved the new HTTP status code 451. Adding to status.py and docs appropriately.

Fixes #3756, adding the name to the HTML title, if it exists.

Update the Django-guardian version for Django 1.9

Add DRF docs to "Documenting your API"

Update bootstrap version in browsable-api.md

Issue 3726 DateTimeField not handling empty values

Raise error when setting a removed rest_framework setting for #3644

Subclass AutoFilterSet from self.default_filter_set

docstring typo fix: DateAndFiles -> DataAndFiles

Move urls.py changes down and add necessary import

Fix tox build against python 3.5 by upgrading pytest.

The previous location of editting urls.py did not allow migrations to be created and the default import needs to be modified

Some minor typos and grammar changes.

Fix transifex config for Django

Version 3.3.2