gh-135763: AC: Implement `allow_negative for Py_ssize_t` (#138150)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Python/modsupport.c

@@ -33,6 +33,19 @@ _Py_convert_optional_to_ssize_t(PyObject *obj, void *result)
     return 1;
 }
 
+int
+_Py_convert_optional_to_non_negative_ssize_t(PyObject *obj, void *result)
+{
+    if (!_Py_convert_optional_to_ssize_t(obj, result)) {
+        return 0;
+    }
+    if (obj != Py_None && *((Py_ssize_t *)result) < 0) {
+        PyErr_SetString(PyExc_ValueError, "argument cannot be negative");
+        return 0;
+    }
+    return 1;
+}
+
 
 /* Helper for mkvalue() to scan the length of a format */