I'm not too sure how I broke the threading tests with this.

As it turns out, the threading aborts are a result of a separate bug, I just managed to trigger it by raising in close. It seems that we don't clean up threads in a subinterpreter properly:

import _interpreters

interp = _interpreters.create()
source = """
import threading

def task():
    time.sleep(100)

t = threading.Thread(target=task)
t.start()
"""
_interpreters.run_string(interp, source)

This causes an assertion failure on my end:

python: Python/pystate.c:1969: tstate_activate: Assertion `!tstate->_status.bound_gilstate || tstate == gilstate_tss_get((tstate->interp->runtime))' failed.

I'm pretty sure that the problem is that even though the interpreter doesn't have a threads.main, the remaining threads.head that finalize_subinterpreters tries to use is still running, and trying to attach to it is a big no-no. The easiest fix would be to call wait_for_thread_shutdown much earlier for subinterpreters (probably where the main interpreter does it), but that doesn't fix anything for C threads, so maybe it's better if we come up with a more robust fix that waits for all threads using a _PySemaphore or something like that. Thoughts, @ericsnowcurrently?

This PR is stale because it has been open for 30 days with no activity.