Sadly there's no way to require Windows and OSX users to run Python > 2.6, since this version is fixed (and always the same) for these platforms:
http://trac.xbmc.org/ticket/7991#comment:22

Linux will include whatever Python version the the system has during the build (2.6 or above).

That said, .format() should use indices to keep compatibility with platforms running Python 2.6.