bpo-38206: Clarify tp_dealloc requirements for heap allocated types. (GH-16248)

As mentioned in the bpo ticket, this mistake came up on two reviews:
- https://github.com/python/cpython/pull/16127#pullrequestreview-288312751
- https://github.com/python/cpython/pull/16071#pullrequestreview-287819525

Would be nice to have it documented in a more permanent place than 3.8's whatsnew entry.


https://bugs.python.org/issue38206



Automerge-Triggered-By: @encukou