Fix typo in Py_DECREF comment

[jkrejcha]

Py_DECREF's comment incorrectly references _Py_IncRef which isn't used by decref. Both functions don't accept NULL but only _Py_DecRef is relevant for the decref function

[ghost]

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[StanFromIreland]

Why change a comment that is wrong to one that is also wrong?

This is explained in a previous comment in the file:
If it may be NULL, use Py_XINCREF/Py_XDECREF instead.

I think this comment should be completely changed or removed.

[picnixz]

The comment is more for internal purposes and implementation details explaining why we are using _Py_DecRef instead of Py_DecRef and for compatibility reasons.

The caller is not meant to pass NULL when using Py_DECREF at all, but the implementation may appear to allow it (at least until Python 3.10).

[jkrejcha]

Indeed this appears to me to be the case. Py_DecRef (notably not Py_DECREF) is just be a thin wrapper around Py_XDECREF.

[picnixz]

@vstinner I plan to merge this one without a commit message and the title as the commit title.

If you however think it's not needed, we can also close this PR (strictly speaking, the comment is also correct but weird considering the implementation and was likely a C/C error).

[vstinner]

LGTM. I confirm that I made a stupid copy/paste mistake here.

[picnixz]

@vstinner In general, we backports docs fixes but by docs I mean docs in pure Python/rst docs. I barely encounter docs fixes at the C level, so do you want to backport this change?

[vstinner]

I don't think that it's worth it to backport such change.