I think it's final time we start considering switching the default PYTHON_SINGLE_TARGET to python3.x. Especially that most of the packages supporting python2 only have only a single python2.7 flag. I think future stabilization of 3.5 is a good point to do the switch. Could you do some testing? If we decide to do it, we should probably also ask releng for some more.
This does not directly affect python-3.5 stabilization.
As of today, PYTHON_SINGLE_TARGET="python3_6" in profiles/base/make.defaults.