You are one of few concerned with Python3 targets.
Pyston, PyPy are all mostly Python(2) projects. PyPy3 does exist, but it's considered beta and not recommended for production use as PyPy is.
Python(2) code/libraries will mainly survive on PyPy/Pyston going forward.
With stuff like Nim and Rusthon, I think something like those will carry the torch forward after Python(2), rather than Python3.
Python(2) code/libraries will mainly survive on PyPy/Pyston going forward. With stuff like Nim and Rusthon, I think something like those will carry the torch forward after Python(2), rather than Python3.