[docs] [issue25701] Document that tp_setattro and tp_setattr are used for deleting attributes

[Martin Panter]

Martin Panter added the comment:

The 2.7 patch looks okay to me. I confirmed that sq_ass_slice gets called to delete slices, and I presume the other stuff is similar to Python 3. Do you want to commit this to 2.7 Serhiy?

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stage: patch review -> commit review

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<http://bugs.python.org/issue25701>
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