Clean up code style.

description.py codes and tests removal

Convert readthedocs link for their .org -> .io migration for hosted projects

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

Wrap guardian.VERSION in tuple, in django-guardian 1.4.2 version has list type and comparison fails. (#4149)

* Use get_serializer_class in ordering filter* Test improper configuration of ordering* Raise ImproperlyConfigured when missing get_serializer_class

* Use get_serializer_class in ordering filter
* Test improper configuration of ordering
* Raise ImproperlyConfigured when missing get_serializer_class

Fix #3844, refine validator for fields with <source=> kwargs

When serializers has fields with something like `source=user.email`, the
uniqueness validator should check `email` field instead of `user`, cause
`user` is a model object.

* added django 1.10 alpha on tox
* added django 1.10 alpha on travis
* added allow_failure on travis matrix

`print` is available as a function on every supported version of Python (2.7+)

Drop out-of-date paragraph on `request.content`

Fix typos in ReturnDict and ReturnList docstrings

ListSerializer fields or serializers that are passed many=True may also take an allow_empty=False argument to disallow empty lists as valid input.

Information about this was part of the 3.2 release announcement, but had not been part of the API docs until now.

Fix incorrectly named zh-hans and zh-hant locale path

Fixes a minor indentation typo.

This allows subclassing TokenAuthentication and setting custom keyword,
thus allowing the Authorization header to be for example:

    Bearer 956e252a-513c-48c5-92dd-bfddc364e812

It doesn't change the behavior of TokenAuthentication itself,
it simply allows to reuse the logic of TokenAuthentication without
the need of copy pasting the class and changing one hardcoded string.

Related: #4080

Fix typo in permissions docs

added python 3.5 checks for django 1.8