Update Demo/parser/unparse.py to current Python 3.x syntax. Additions:

- relative imports
 - keyword-only arguments
 - function annotations
 - class decorators
 - raise ... from ...
 - except ... as ...
 - nonlocal
 - bytes literals
 - set literals
 - set comprehensions
 - dict comprehensions
Removals:
 - print statement.

Some of this should be backported to 2.x.