[docs] [issue17351] Fixed python3 descriptor documentation example + removal of explicit "object" inheritance in docs

[Roundup Robot]

Roundup Robot added the comment:

New changeset 513c4aaf70d7 by Raymond Hettinger in branch '2.7':
Issue #17351: Modernize the pure Python property() example.
http://hg.python.org/cpython/rev/513c4aaf70d7

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nosy: +python-dev

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