[mypyc] Fix memory safety of list.count on free-threaded builds (#21680)

Work on mypyc/mypyc#1202.

Author: JukkaL

mypyc/lib-rt/pythonsupport.h

@@ -205,13 +205,33 @@ list_count(PyListObject *self, PyObject *value)
     Py_ssize_t count = 0;
     Py_ssize_t i;
 
+#ifdef Py_GIL_DISABLED
+    for (i = 0; i < PyList_GET_SIZE(self); i++) {
+        PyObject *item = PyList_GetItemRef((PyObject *)self, i);
+        if (unlikely(item == NULL)) {
+            // Race condition: list shrank between size read and get item
+            if (PyErr_ExceptionMatches(PyExc_IndexError)) {
+                PyErr_Clear();
+                break;
+            }
+            return CPY_INT_TAG;
+        }
+        int cmp = PyObject_RichCompareBool(item, value, Py_EQ);
+        Py_DECREF(item);
+        if (cmp > 0)
+            count++;
+        else if (cmp < 0)
+            return CPY_INT_TAG;
+    }
+#else
     for (i = 0; i < Py_SIZE(self); i++) {
         int cmp = PyObject_RichCompareBool(self->ob_item[i], value, Py_EQ);
         if (cmp > 0)
             count++;
         else if (cmp < 0)
             return CPY_INT_TAG;
     }
+#endif
     return CPyTagged_ShortFromSsize_t(count);
 }