[docs] [issue28771] Update documented signatures of tp_get/setattr

[Martin Panter]

Martin Panter added the comment:

Python 2 patch is on top of the patch for Issue 25701

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dependencies: +Document that tp_setattro and tp_setattr are used for deleting attributes

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28771>
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