Change str.format() to support python 2.6

While I can certainly sympathize with the fact that maintaining older version is a pain,  this is a very small change that enables the lib to be used with python 2.6 (tested with 2.6.6 at least).

This is especially important since CentOS 6 is still widely deployed and it's stuck with python 2.6, so we have no choice but to use it.