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>
The internal encoder object returned by undocumented function
json.encoder.c_make_encoder() (aka _json.make_encoder()) crashed
when it was called with non-zero second argument.
