qstr, objstr: Make sure that valid hash != 0, treat 0 as "not computed".

This feature was proposed with initial hashing RFC, and is prerequisite for
seamless static str object definition.
objstr: Allow to define statically allocated str objects.

Similar to tuples, lists, dicts. Statically allocated strings don't have hash
computed.
modsys: Implement sys.version.

The logic appears to be that (at least beginning of) sys.versions is the
version of reference Python language implemented, not version of particular
implementation.

Also, bump set versions at 3.4.0, based on @dpgeorge preference.