[docs] [issue17351] Remove explicit "object" inheritance in Python 3 docs

[Michael Foord]

Michael Foord added the comment:

If the consensus is that Python documentation should not include explicit object inheritance then fine to update mock docs accordingly. The backport docs will still use it as they are for Python 2 as well.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17351>
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