bpo-40881: Fix unicode_release_interned() (GH-20699)

Use Py_SET_REFCNT() in unicode_release_interned().

Author: vstinner

Objects/unicodeobject.c

@@ -15669,13 +15669,13 @@ unicode_release_interned(void)
         }
         switch (PyUnicode_CHECK_INTERNED(s)) {
         case SSTATE_INTERNED_IMMORTAL:
-            Py_REFCNT(s) += 1;
+            Py_SET_REFCNT(s, Py_REFCNT(s) + 1);
 #ifdef INTERNED_STATS
             immortal_size += PyUnicode_GET_LENGTH(s);
 #endif
             break;
         case SSTATE_INTERNED_MORTAL:
-            Py_REFCNT(s) += 2;
+            Py_SET_REFCNT(s, Py_REFCNT(s) + 2);
 #ifdef INTERNED_STATS
             mortal_size += PyUnicode_GET_LENGTH(s);
 #endif