[Rivet] SL4 compatibility

[Frank Siegert]

Hi guys,

Just wondering, whether we can be a bit more careful with the python code, 
to make sure it is compatible with SL4, which only has Python 2.3?
Obviously we are all Ubuntuers nowadays, but our users certainly need this 
on SL4 (and I too, on the Grid).

I have just tried to fix bin/rivet and bin/agile-runmc, but for both at 
least one minor issue is remaining (set() and sorted() not available in 
2.3).

Thanks,
Frank