[Rivet] python2 change

[David Grellscheid]

Hi Andy,

> Being able to assume Python 2.6.6 or later would make the migration far
> easier, I think. I'm personally happy to drop support for SL5 now: the
> whole Grid has managed to switch to SL6 so individuals have ~no excuse!

I agree with dropping SL5. Python 2.6 came out in 2008 and makes the 
transition very easy with many "from __future__" helpers already provided.

> I don't know if there are many "standard" systems out there with Python
> 2.5. What versions are installed on current/recent Macs?

Mavericks has 2.7.5, so no problem there. Mac owners typically upgrade 
fairly quickly.

   David