libpsl, python and dependecies

[Vincent Habchi]

Josh,

> The point is just that creating the variants is one thing, and knowing what to do when they are set is quite another. There's no generic python variant body that will work in all cases.

That’s why I’m planning to provide a basic set of functions, and then a hook point (a specific named procedure) for the Port to run extra commands that might be needed.

Vincent