Incorrectly ordered imports using tox
<p>I am trying to make a contribution to django-rest-framework, the imports in authentication file after I run <code>isort</code> are like this(I have added import six):</p>

<pre><code>from __future__ import unicode_literals

import base64

import six
from django.contrib.auth import authenticate, get_user_model
from django.middleware.csrf import CsrfViewMiddleware
from django.utils.translation import ugettext_lazy as _

from rest_framework import HTTP_HEADER_ENCODING, exceptions
</code></pre>

<p>when I run <code>./runtests --lintonly</code> it passes all the tests but when I run <code>tox</code> it gives me this error:</p>

<pre><code>py27-lint runtests: commands[0] | ./runtests.py --lintonly
Running flake8 code linting
flake8 passed
Running isort code checking
ERROR: /home/nitesh/open_source/django-rest-framework/rest_framework/authentication.py Imports are incorrectly sorted.
isort failed: Some modules have incorrectly ordered imports. Fix by running `isort --recursive .`
ERROR: InvocationError: '/home/nitesh/open_source/django-rest-framework/runtests.py --lintonly'
</code></pre>