Are you sure this is a python issue and not an OS issue?  That is, does an equivalent C program work correctly?  Since the operation is a fork, I don't think there's anything that python does that would cause the signal to be ignored.

The comment block in the example code here: https://www.win.tue.nl/~aeb/linux/lk/lk-5.html makes me think that the signal getting ignored is a possibility at the OS level, though it isn't explicitly clear.