documentation/python: implement context-relative linking.

documentation/python: implement context-relative linking.

I didn't change much, but the amount of research needed to verify that I'm
indeed doing it (mostly) correct was extreme. There's a pathological case
inside a named subclass where I'm doing it differently than Python itself,
but I don't expect this to be hit very often, so postponing until later.

The tests also assert inside the function definitions (and those functions
get called) to verify we're indeed doing (mostly) correct stuff.