[issue12647] Add __bool__ to None

[STINNER Victor]

STINNER Victor <victor.stinner at haypocalc.com> added the comment:

> I'm not sure why the special case shouldn't be retained.

We can keep the fast path in PyObject_IsTrue but add a __bool__ method to NoneType.

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nosy: +haypo

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<http://bugs.python.org/issue12647>
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