2to3: Skip `setliteral` fixer.

Setliterals are not available in Python 2.6. Because the current usage
in numpy is forward compatible with Python 3 there is no need to run the
fixer.

When Python 2.6 support is dropped the fixer can be run, but that looks
to be several years off. RHEL 6 with Python 2.6 was released in 2010 and
will run for ten years.