gh-37194: Fix typo in pycore_long.h for Python <= 3.8.

    
This fixes the same issue as in
sagemath/cypari2#146.

Typo was added in dc71bd0.
    
URL: #37194
Reported by: Ján Jančár
Reviewer(s): Gonzalo Tornaría, Sebastian Spindler

src/sage/cpython/pycore_long.h

@@ -89,7 +89,7 @@ _PyLong_SetSignAndDigitCount(PyLongObject *op, int sign, Py_ssize_t size)
 {
 #if (PY_MAJOR_VERSION == 3) && (PY_MINOR_VERSION < 9)
 // The function Py_SET_SIZE is defined starting with python 3.9.
-    Py_SIZE(o) = size;
+    Py_SIZE(op) = size;
 #else
     Py_SET_SIZE(op, sign < 0 ? -size : size);
 #endif