gh-129643: Fix PyList_Insert in free-threading builds (#129680)

Author: sobolevn

Misc/NEWS.d/next/Core_and_Builtins/2025-02-05-11-29-52.gh-issue-129643.4mGzvg.rst

@@ -0,0 +1 @@
+Fix thread safety of :c:func:`PyList_Insert` in free-threading builds.

Objects/listobject.c

@@ -466,8 +466,8 @@ ins1(PyListObject *self, Py_ssize_t where, PyObject *v)
         where = n;
     items = self->ob_item;
     for (i = n; --i >= where; )
-        items[i+1] = items[i];
-    items[where] = Py_NewRef(v);
+        FT_ATOMIC_STORE_PTR_RELAXED(items[i+1], items[i]);
+    FT_ATOMIC_STORE_PTR_RELEASE(items[where], Py_NewRef(v));
     return 0;
 }