[bug] fix issue #23

on macOS spawn is used as the default multiprocessing context instead of fork since Python 3.8

this means that if a module is read from a .py file using the runpy method, my workaround from ab81df8a doesn't work because the shimmed sys.modules entry will not exist in the spawned process (as it would with fork) and therefore unpickling functions from the hook module fails

if we re-run HookManager._load_module in the worker process initialization (which runs before unpickling anything else) we can re-install the shim and everything goes fine