gh-142831: Fix use-after-free in json encoder during re-entrant mutation (gh-142851)

Hold strong references to borrowed items unconditionally (not only in                                                                        
free-threading builds) in _encoder_iterate_mapping_lock_held and                                                                             
_encoder_iterate_fast_seq_lock_held.  User callbacks invoked during                                                                          
encoding can mutate or clear the underlying container, invalidating                                                                          
borrowed references.                                                                                                                         
                                                                                                                                               
The dict iteration path was already fixed by gh-145244.

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
Co-authored-by: Gregory P. Smith <greg@krypto.org>