MemoryError freelist is not thread-safe in free threaded build

[colesbury]

Bug report

The MemoryError freelist isn't thread-safe if the GIL is disabled:

cpython/Objects/exceptions.c

Lines 3850 to 3860 in 285c1c4

	/* Fetch object from freelist and revive it */
	self = state->memerrors_freelist;
	self->args = PyTuple_New(0);
	/* This shouldn't happen since the empty tuple is persistent */
	if (self->args == NULL) {
	return NULL;
	}
	state->memerrors_freelist = (PyBaseExceptionObject *) self->dict;
	state->memerrors_numfree--;

Most of the freelists were made thread-safe by making them per-thread in the free threaded build (using pycore_freelist.h), but we don't want to do that for MemoryError because its freelist serves a different purpose (it's not really for performance). I think we should just use a lock for MemoryError's freelist.

Linked PRs

• gh-129668: Fix thread-safety of MemoryError freelist in free threaded build #129704
• [3.13] gh-129668: Fix thread-safety of MemoryError freelist in free threaded build (gh-129704) #129742