test: Add refence to the issue in the test comment.

Co-authored-by: Peter Bierma <[email protected]>

Author: Mr-Sunglasses

Lib/test/test_gzip.py

@@ -468,7 +468,7 @@ def sizes():
             self.assertEqual(nread, len(uncompressed))
 
     def test_no_refloop_with_writestream(self):
-        """Test that the _WriteBufferStream __del__ method prevents reference loops."""
+        """GH-129640: Test that the _WriteBufferStream __del__ method prevents reference loops."""
         import gc
 
         gc.collect()