Don't set use_2to3 unless we have to.

This is a minor point.  If the user has an old version of distribute installed (such as the 0.6.11c that ships with OS-X Mountain Lion) it doesn't have the use_2to3 flag.  This results in a Warning about an unknown keyword argument.  It's not needed on Python 2 anyway, so it's easiest to just not include it on Python 2.

Note that this is irrelevant for master, where 2to3 is no longer used.