Since it breaks for anything non-ascii, its not that helpfull after all
and since python2 strings are encoding-unaware there is no way to fix
it.

It might be preferable to supply unpicklers that are cappable of
coercing if the user really wants wants coercing.

yup
> 
> > 3. python 3 string map to python 2 unicode 
> 
> That's also the case, AFAICT.
yup
> 
> > 4. python 3 bytestring maps to python 2 string
> 
> Hmm. This may be indeed a mistake. Until r61467, bytes were saved
> with the (BIN)STRING code; not sure why this was changed.
Python 3 is indeed evil there.

b'\x80\x02c__builtin__\nbytes\nq\x00]q\x01\x85q\x02Rq\x03.'

I'm convinced that a 1:1 mapping of python2 string from/to python3
bytestrings is the least surprising behaviour and will keep surprising
errors away when needing to communicate between different python
versions.

It just has bitten me, and i suspect will will get others, too.
Unpickle that completely fails in the face of encodings is not desirable
at all.