Test fixes for Py3, Django changes

* tests:
    * use skipIf for digest, oauth tests to avoid Py3 failures
* tox:
    * split py2/py3 python-digest releases
    * Created tox deps-py2 / deps-py3 variables for each env to include
    * Moved python3-digest and oauth2 from requirements.txt to appropriate deps
    * Remove python 2.6 / django-dev target
* Remove remaining deprecated django.utils.simplejson usage
* Fix docs build