Investigate HPy API proposal
The HPy API was presented recently at the Python Language Summit (LWN article and PSF article). This is proposed as a way to make Python extensions with compiled code that are agnostic to the Python interpreter in use. If true, this would mean we'd be able to compile once and work with CPython 3.X (once support lands), CPython 3.X+1, and PyPy all at once (if I'm reading it right).
It also adds a ctx parameter (similar to Lua's L argument) which would mean we could keep different subsystems using separate Python interpreters (finally!).
This is mostly meant as an awareness/tracking/planning issue; HPy is not yet complete and there are no plans to port VTK to it, but it can't hurt to think about it.