A pickled str instance written by 2.6 currently unpickles under 3.0 as a
bytes instance. That would be correct if the intended use is binary
data, but it's wrong if the intended use is text. My hunch is that
there's more pickled text than binary data. (E.g. a dict containing
instance variables uses (8-bit) str instances for the keys; these *must*
be unpacked as (Unicode) str instances in 3.0.)

The inverse issue also exists.

We need to DECIDE this before starting to code (coding is probably
minimal).  I'm assigning the task to DECIDE (after discussion on the
list) to myself.