Make sitescripts (the repo) compatible with Python 3

[kvas]

Background

Python 2 will cease being supported at the start of 2020. We have decided to migrate the code and deployments to Python 3 before that date. The parts of sitescripts that are still in use will need to be migrated.

What to change

Make deployed parts of sitescripts compatible with Python 3 (while retaining Python 2 compatibility). It's also possible to instead separate some part into a separate project that would be a Python 3 only python package and then update the deployment setup.

The first step would be to identify deployed parts of sitescripts together with Ops and make a plan for each part (make compatible or split out). After that each part can be deal with in a separate ticket.

Ideally we would like to have tests for parts that involve non-mechanical changes, particularly in the split out scenario (we don't want to create new legacy code).

[kvas]

This work is now documented ​here and tracked in the GitLab tracker.