[Cython] Implemented Set/FrozenSet support for Python 2.3

[Stefan Behnel]

Hi,

Lisandro Dalcin wrote:
> #if PY_VERSION_HEX < 0x02050000
> #ifndef PyAnySet_CheckExact
> 
> /* COMMON STUFF FOR Py 2.3 and 2.4 */
> 
> #endif /* PyAnySet_CheckExact (<= Py2.4) */
> 
> Why are you protecting this stuff with the PyAnySet_CheckExact definition ??

Just because that's something that was not available in the old Py2.4 set
implementation (which is different from the one in 2.5+, BTW). So that's a way
to distinguish between a 'real' 2.5 and, say, a 2.4 with an updated set
implementation. From the back of my head, I kind of remember murmurs about a
backport at the time, but maybe that was for 2.3...

I admit that it's also a very rare thing to have a non-standard Python build,
so I guess this can be dropped together with the error checks...

Stefan