An example of action-at-a-distance: fix the problems I had in test_io.py

An example of action-at-a-distance: fix the problems I had in test_io.py
without touching io.py or test_io.py.  The cause of the failure was that
bytes objects didn't pickle right.  As a stop-gap measure, I'm providing
bytes pickling via copy_reg.  Eventually, we should use a more efficient
protocol, e.g. __reduce_ex__ or __getstate__/__setstate__.