Support Django 3.0+

[wm3ndez]

django.utils.six and some aliases were removed from Django in 3.0.

[coveralls]

Coverage remained the same at ?% when pulling 561563c on wm3ndez:master into 22ccd97 on jazzband:master.

[coveralls]

Coverage remained the same at ?% when pulling d1ab97e on wm3ndez:master into 22ccd97 on jazzband:master.

[dmptrluke]

Since all of the functions in six only exist to provide Python 2 and 3 compatibility, it may be a better approach to check over the six documentation and replace the all usages of six functions with their native Python 3 versions.

For example, six.StringIO is just an alias for io.StringIO on Python 3, so we can use that directly. Another example is six.text_type, which is just an alias for the str type.

[hugodes]

This would solve #601

[wm3ndez]

@dmptrluke Yeah I think it makes sense to just remove PY2 support now. Will do that.

[dmptrluke]

We need to remove Programming Language :: Python :: 2.7 from setup.py, and probably add Django 3.0 to the readme, but it looks good!

[wm3ndez]

@dmptrluke Done. Thanks!

[wm3ndez]

@dmptrluke Should I merge the PR?

[dmptrluke]

I'm not sure what the best approach is - this is my first time contributing to this project, so I'm not sure who has permission to perform PyPi releases - we probably need to get them involved so we can properly publish these changes as a new version once merged.

[camilonova]

@wm3ndez Thank you!