> the basic behavior i want to see for all protocols <= 2
> 
> 1. python 2 string maps to python3 byte-string

That would not be good. Many people create pickles in 2.x where the
string type really represents characters, more often so than they want
it to represent bytes. Giving them bytes on unpickling will likely
cause more problems than the current approach.

> 2. python 2 unicode maps to python3 string

That's the case, right?

> 3. python 3 string map to python 2 unicode 

That's also the case, AFAICT.

> 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.