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The example object in the xxlimited module can be part of a reference loop (it can contain itself), so it needs GC and tp_traverse.
The tp_dealloc hook was incorrect, and a correct version would be difficult to generalize for something more complicated than a example class (#16690; [0]). It's better to avoid dealloc and show off tp_finalize.
Same for the class in _testmultiphase (PEP 489 tests), which is based on xxlimited. The incorrect dealloc is causing the reference leak mentioned in #24268.
The Xxo object also wasn't actually added to the module.
Here is a patch to fix these.
[0] https://mail.python.org/pipermail/python-dev/2015-June/140422.html 

Would it also be worth making at docs update to tp_dealloc, suggesting the use of tp_traverse/finalize?: https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_dealloc
And perhaps from PyType_FromSpec? https://docs.python.org/3/c-api/type.html?highlight=pytype_fromspec#c.PyType_FromSpec 

tp_traverse is completely orthogonal to tp_dealloc, it's needed to detect (and then break) reference cycles like:
 obj = xxlimited.Xxo()
 obj.foo = obj
As for tp_finalize: yes, mentioning it in tp_dealloc docs would be good, but I'll need a bit more studying to understand the problem correctly. The cases fixed here are relatively simple; Antoine gives more complex ones in [0]. When I feel qualified to give advice, I'll change the docs. (And most likely, write a PEP to make things easier; some changes to classes will be needed anyway to make PEP 489 multi-phase init work nicely in all cases).
But, I plan to focus my CPython time on documenting PEP 489 before diving in here. I think issue 16690 is a good place to track tp_dealloc docs changes.
[0] https://mail.python.org/pipermail/python-dev/2015-June/140423.html 

New changeset 265eeb60443a by Nick Coghlan in branch '3.5':
Issue #24373: Eliminate PEP 489 test refleaks
https://hg.python.org/cpython/rev/265eeb60443a
New changeset f24cd8bc5250 by Nick Coghlan in branch 'default':
Merge fix for issue #24373 from 3.5
https://hg.python.org/cpython/rev/f24cd8bc5250 

tp_finalize handlers should return "void". See issue33644.