Support iterators that return bytes.
Support iterators that return bytes.

The assertions are copied from https://source.corp.google.com/piper///depot/google3/third_party/clif/python/pyext.py;l=831.

Manually tested with https://source.corp.google.com/piper///depot/google3/quality/ranklab/future_oss/data_structures/python/vector_wrapper.clif;l=126.

PiperOrigin-RevId: 444405953

google/clif,google/clif,google/clif