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Didn't have much sleep last night, so please forgive me if I say something stupid, but:
Python/pystate.c:
void
PyThreadState_DeleteCurrent()
{
	PyThreadState *tstate = _PyThreadState_Current;
	if (tstate == NULL)
		Py_FatalError(
			"PyThreadState_DeleteCurrent: no current tstate");
	_PyThreadState_Current = NULL;
	tstate_delete_common(tstate);
	if (autoTLSkey && PyThread_get_key_value(autoTLSkey) == tstate)
		PyThread_delete_key_value(autoTLSkey);
	PyEval_ReleaseLock();
}
the current tstate is deleted and freed before releasing the GIL, so if another thread calls take_gil after the current thread has called tstate_delete_common but before it calls PyEval_ReleaseLock (which calls drop_gil and set gil_locked to 0), then it will enter this section and dereference gil_last_holder.
I just checked with valgrind, and he also reports an illegal dereference at this precise line.