gh-111178: Fix function signatures in odictobject.c

[vstinner]

• Issue: UBSan: Calling a function through pointer to incorrect function type is undefined behavior #111178

[picnixz]

If you want to also make similar changes as I did in my PRs, for the NOARGS method, I renamed the ignored parameter to dummy and for METH_O ones, I named it arg so that by reading the function itself, we can know how it's used (I found "ignored" confusing as we already have Py_UNUSED...).

If you find it bothersome or don't think it adds value, feel free to ignore this suggestion.

[vstinner]

If you want to also make similar changes as I did in my PRs, for the NOARGS method, I renamed the ignored parameter to dummy and for METH_O ones, I named it arg so that by reading the function itself, we can know how it's used (I found "ignored" confusing as we already have Py_UNUSED...).

I'm fine with "ignored", so I prefer to leave these parameters as they are :-)