[Python-3000] Immutable bytes type and dbm modules

["Martin v. Löwis"]

> Yeah you did the keys (good!).  I just checked in a change to require
> values to also by bytes.  Maybe that goes so far as to be inconvenient?

Ah, ok. I think it is fine. We still need to discuss what the best
way is to do string:string databases, or string:bytes databases.

I added StringKeys and StringValues to allow for such cases, and I
also changed shelve to use string keys (not bytes keys), as this
is really a dictionary-like application; this all needs to be
discussed.

Regards,
Martin