27/08/2019: Python 2 will never die
When developing KISS I wanted to ship Python 3 and try to avoid the dependence on Python 2. This ended up becoming an impossibility due to two offending packages, Firefox and NodeJS (a build dependency of the former).
We are nearing the day that Python 2 reaches EOL (end of life) and as is usually the case with deadlines, it has been left to the last minute. Python 2's end has been known as far back as 2010 (possibly sooner) and yet I foresee it being used well past 2020. 
Firefox's build process depends on both Python 2 and Python 3. Mozilla is currently in the process of converting their Python code to support both 2 and 3.
This effort only recently started and there is a long road ahead to completion.
In mozilla-central there are over 3500 Python files (excluding third party files), comprising roughly 230k lines of code. Additionally there are 462 repositories labelled with Python in the Mozilla org on Github [...] 
Will Mozilla finish before the deadline? It doesn't look that way.
Do we need to be 100% migrated by Python 2’s EOL? Technically, no. [...] 
Node is in a similar position to Mozilla, work is being done but to say it is nearing completion is an overstatement. NodeJS is closer to being Python 3 compatible than Firefox but I myself doubt that we will see a release with support by 2020.
It is looking more and more like we will be using Python 2 for a while longer. Despite the decade or near decade long time for transition these projects have left the transition till beyond the EOL date.
Since the EOL announcement till today how much Python 2 code has been added to these two projects? A transition even 5 years ago would've been beneficial!
So it looks like KISS will keep Python 2 in the official repositories past 2019 and into 2020.
-  https://mail.python.org/pipermail/python-dev/2010-May/099971.html
-  https://ahal.ca/blog/2019/python-3-at-mozilla/