For anyone confused, currently you have a choice of either "Standard Environment", which is Python 2.7 only, or "Flexible Environment" which allows you to choose Python 2.7, Python 3.6, or your own Python runtime [1][2]. This is about supporting Python 3 in the "Standard Environment". It's unclear whether the intention is to replace Python 2 or allow a choice?

[1] https://cloud.google.com/appengine/docs/python/ [2] https://cloud.google.com/appengine/docs/flexible/python/runt...