try to import guardian if it's in INSTALLED_APPS

Fix codecov on Travis CI

The coverage report needs to be explicitly created with arguments passed
in to pytest-cov

Added django-rest-framework-social-oauth2 to list of third party packages

Use serializers.ValidationError in AuthTokenSerializer

Small doc typo `throttles` --> `throttling`

Per django rest framework docs, and to prevent confusion with Django's ValidationError, `serializers.ValidationError` is preferred to `exceptions.ValidationError`.

Added drf-haystack to third party resources

Fixed minor doc typo.

Added JSONField.

Remove content overriding

Django 1.9 support

Drop django 16

Merge branch 'drop-django-16' oft https://github.com/carltongibson/django-rest-framework into carltongibson-drop-django-16

Conflicts:
	docs/topics/release-notes.md

Compat updates for version 3.3

Conflicts:
	docs/topics/release-notes.md

Access settings lazily, not at module import.

Use model error_messages when available

fixing a trivial HTML tag which is closed twice

In the automatically applied UniqueValidator, use the error message from
error_messages defined in the model instead of the generic default
UniqueValidator message.

This fixes #2878.

Guard against calling `serializer.data` before `serializer.save()`

Release/3.2.4

- Asserts that `_data` does not exist when calling `.save()`

- Added test_data_access_before_save_raises_error test