gh-99845: Clean up _PyObject_VAR_SIZE() usage

[vstinner]

• code_sizeof() now uses unsigned type to compute the result.
• Fix _PyObject_ComputedDictPointer(): cast _PyObject_VAR_SIZE() to Py_ssize_t, rather than long.
• Clarify that _PyObject_VAR_SIZE() uses an unsigned type (size_t).

• Issue: PEP 670: Convert _PyObject_SIZE() and _PyObject_VAR_SIZE() macros to functions #99845

[serhiy-storchaka]

It should be backported.

[vstinner]

The dictoffset code was different in 2001 when the cast was added: commit 6d483d3

Change:

-               dictoffset += size;
+               dictoffset += (long)size;

Code:

PyObject **
_PyObject_GetDictPtr(PyObject *obj)
{
        long dictoffset;
        ...
        if (dictoffset < 0) {
                size_t size;
                _PyObject_VAR_SIZE(size, tp, ((PyVarObject *)obj)->ob_size);
                dictoffset += (long)size;
                assert(dictoffset > 0);
                assert(dictoffset % SIZEOF_VOID_P == 0);
        }
        ...
}

The dictoffset type was changed in 2006 by commit 725507b without updating the cast, forgotten in the enhancement.

My change, replace (long) cast with (Py_ssize_t) only impact 64-bit Windows. On other platforms, long and Py_ssize_t should be the same type (either 32-bit or 64-bit signed integer, int32_t or int64_t).

[vstinner]

I created PR #99922 to backport the cast fix to Python 3.11 (and 3.10).