Port to Python 3

[julen]

Let's make sure zing runs on Python 3. This will also eventually drop support for Python 2 because some dependencies with support for security fixes, namely Django, only run on Python 3.

• Upgrade to py2- & py3-compatible dependencies as much as possible (Upgrade dependencies to py3-compatible versions #378)
• Make sure tests run on py3
• Update code to py3 (https://docs.python.org/3/howto/pyporting.html)
• Drop Python 2 support
• Upgrade to Django 2 (Upgrade to Django 2 #399)
• Upgrade to Django 3 (Upgrade to Django 3 #400)
• Upgrade library dependencies to their latest versions

[julen]

Closing since the gross of the migration has been done, the rest will be ad-hoc follow-up fixes.