(Previously, protocol 2 was used for Python 2.)

  The zodbpickle package provides a `zodbpickle.binary` string type
  that should be used in Python 2 to cause binary strings to be saved
  in a pickle binary format, so they can be loaded correctly in
  Python 3.  Pickle protocol 3 is needed for this to work correctly.

- Object identifiers in persistent references are saved as
  `zodbpickle.binary` strings in Python 2, so that they are loaded
  correctly in Python 3.