gh-140061: Use _PyObject_IsUniquelyReferenced() to check if objects are uniquely referenced (gh-140062)

The previous `Py_REFCNT(x) == 1` checks can have data races in the free
threaded build. `_PyObject_IsUniquelyReferenced(x)` is a more conservative
check that is safe in the free threaded build and is identical to
`Py_REFCNT(x) == 1` in the default GIL-enabled build.

Python/marshal.c

@@ -11,6 +11,7 @@
 #include "pycore_code.h"             // _PyCode_New()
 #include "pycore_hashtable.h"        // _Py_hashtable_t
 #include "pycore_long.h"             // _PyLong_IsZero()
+#include "pycore_object.h"           // _PyObject_IsUniquelyReferenced
 #include "pycore_pystate.h"          // _PyInterpreterState_GET()
 #include "pycore_setobject.h"        // _PySet_NextEntryRef()
 #include "pycore_unicodeobject.h"    // _PyUnicode_InternImmortal()
@@ -388,7 +389,7 @@ w_ref(PyObject *v, char *flag, WFILE *p)
      * But we use TYPE_REF always for interned string, to PYC file stable
      * as possible.
      */
-    if (Py_REFCNT(v) == 1 &&
+    if (_PyObject_IsUniquelyReferenced(v) &&
             !(PyUnicode_CheckExact(v) && PyUnicode_CHECK_INTERNED(v))) {
         return 0;
     }