[Python-Dev] Force to use Py_CLEAR and Py_VISIT in example for Python extending

[Andrew Svetlov]

For now http://docs.python.org/3/extending/newtypes.html#supporting-cyclic-garbage-collection
at first the doc demonstrate long way than note Py_CLEAR and Py_VISIT macroses.

I like to remove code similar to

    if (self->first) {
        vret = visit(self->first, arg);
        if (vret != 0)
            return vret;
    }

and

    tmp = self->first;
    self->first = NULL;
    Py_XDECREF(tmp);

and replace those to Py_VISIT and Py_CLEAN.

I think we have to demonstrate best practices in our examples.
Let's py2.7 branch live untouched, change only docs for python 3.x

Any objections?


--
Thanks,
Andrew Svetlov