Retract the claim that this is always safe if PyMem_{Del, DEL, Free, FREE}

are called without the GIL.  It's incredibly unlikely to fail, but I can't
make this bulletproof without either adding a lock for exclusion, or
giving up on growing the arena base-address vector (it would be safe if
this were a static array).

Objects/obmalloc.c

@@ -403,6 +403,10 @@ new_arena(void)
 		 * only make a supposed-to-succeed case fail by mistake).
 		 * Read the above 50 times before changing anything in this
 		 * block.
+		 * XXX Fudge.  This is still vulnerable:  there's nothing
+		 * XXX to stop the bad-guy thread from picking up the
+		 * XXX current value of arenas, but not indexing off of it
+		 * XXX until after the PyMem_FREE(oldarenas) below completes.
 		 */
 		uptr *oldarenas;
 		int newmax = maxarenas + (maxarenas >> 1);