Remove dependency from django.utils.six

[rbialon]

This dependency is used only for one import, which is now included with
different sources for Python 2.x and 3.x, respectively.

With Python 3, the StringIO module is gone and all functionality is
included in io
(See https://docs.python.org/3.0/whatsnew/3.0.html#text-vs-data-instead-of-unicode-vs-8-bit )

This PR closes #162

[rbialon]

The failing tests seem to be caused by dropped Python 3.4 support from Pillow python-pillow/Pillow#3581

[Pyvonix]

I will be happy to see this applied to start working on Django 3.0

As @rbialon said and as we can read in the official doc:

Django 3.0 supports Python 3.6, 3.7, and 3.8. We highly recommend and only officially support the latest release of each series.
The Django 2.2.x series is the last to support Python 3.5.

Did we really need to support Python 3.4? Because in Django 2.X, the minimum require is 3.5... So can we consider Python 3.4 as little bit outdated?

[rbialon]

As Python 3.4 was released seven years ago and has reached its end-of-life in March 2019, I would say it's deprecated, too.

[Pyvonix]

May be you can remove the Travis job for Python 3.4 and create news jobs for Python 3.7 and Python 3.8 ? It's could be the moment to add some jobs for Django 2.2 and Django 3.0?

Fallowing the Django documentation, to know which version of Django is recommend with which version of Python. Like this, all checks will be green and the MR could be easily validate in the web interface (may be)

[adi-]

Fixing this in own commit. Stay tuned.