Issue #13992: The trashcan mechanism is now thread-safe. This eliminates sporadic crashes in multi-thread programs when several long deallocator chains ran concurrently and involved subclasses of built-in container types. Because of this change, a couple extension modules compiled for 2.7.4 (those which use the trashcan mechanism, despite it being undocumented) will not be loadable by 2.7.3 and earlier. However, extension modules compiled for 2.7.3 and earlier will be loadable by 2.7.4.