Use Python 3 compatible MySQL database adapter

It's recommended in the Django docs:
https://docs.djangoproject.com/en/dev/ref/databases/#mysql-notes

A workaround is needed to avoid a KeyError when using with Django 1.7.
Disallow failing tests with Python 3 and Django 1.7

This marks the end of my work on Django 1.7 and Python 3. Support for
both can now be considered stable.