Minor import cleanups

boxtree/tree_build.py

-from __future__ import division
-from __future__ import absolute_import
-from six.moves import range
-from six.moves import zip
+from __future__ import division, absolute_import
 
 __copyright__ = "Copyright (C) 2012 Andreas Kloeckner"
 
@@ -26,6 +23,8 @@ THE SOFTWARE.
 """
 
 
+from six.moves import range, zip
+
 import numpy as np
 from pytools import memoize_method
 import pyopencl as cl