[issue19692] Rename Py_SAFE_DOWNCAST

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

While Py_SAFE_DOWNCAST is not documented, it doesn't start with underscore.
How many 3rd party code are broken by changing the name?

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nosy: +inada.naoki

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue19692>
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