COMPAT: drop support for python 2.6

[cpcloud]

will close #8760 at same time

The slowest of the slow of Linux distributions (RHEL) has upgraded their version of Python to 2.7. I think if they're happy with not supporting 2.6, then we can be too. Supporting it is not a huge deal, but it would be nice to really encourage people to move forward with upgrading to later versions of Python which would provide the community with momentum towards Python 3. While we can't force everyone to use Python 3, I think dropping support for Python 2.6 is reasonable. The last binary release of Python 2.6 was 2.6.6 and that was in 2010. By now, most people are using python 2.7.

[jreback]

let's wait / defer the decision on this till next version. I don't think this is a problem (until numpy < 1.7)

[cpcloud]

Cool sounds good