gh-130957: Use sleeping_retry in test_free_reference

The weak reference may not be immediately dead.

Author: colesbury

Lib/test/test_concurrent_futures/executor.py

@@ -129,4 +129,7 @@ def test_free_reference(self):
             wr = weakref.ref(obj)
             del obj
             support.gc_collect()  # For PyPy or other GCs.
-            self.assertIsNone(wr())
+
+            for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
+                if wr() is None:
+                    break