Back around python 1.5, there was almost a fork of python where every object had locks, there were memory arenas, and multiprocessing was almost thoughtlessly easy. That and stackless would've been great.
More recently than that, too, unless I'm misremembering. As sibling commenter points out, such a model is horrifically slow. Lock-for-every-object is too granular to perform well.
IronPython did that too, on .Net. It ran around one quarter the speed of CPython.