Please disregard my comment on PyEval_ReInitThreads and _after_fork:
it will of course still be necessary, because it does much more than
just reinitializing locks (e.g. stop threads).
Also, note that both approaches don't handle synchronization
primitives other  than bare Lock and RLock. For example, Condition and
Event used in the threading module wouldn't be reset automatically:
that's maybe something that could be handled by Gregory's atfork
mechanism.