[Cython] Implemented Set/FrozenSet support for Python 2.3

[Stefan Behnel]

Hi again,

Stefan Behnel wrote:
> Lisandro Dalcin wrote:
>> 1) You have a 'cython.py' script. This script just add 'set' and
>> 'frozenset' to __builtin__ module for the case of Python 2.3.
> 
> IMHO, the compiler shouldn't mingle with __builtins__, so this must be handled
> directly in Cython as a special case.

I just noticed that that special casing code is more or less there already in
Builtin.py. I'll see if I can fix your patch up.

Aaron, given that you requested this feature, could you come up with a good
doctest for sets? See tests/run/*.pyx for examples on how it's done.

Stefan