remove problematic import

nifty/field.py

@@ -18,7 +18,7 @@
 
 from __future__ import division
 from builtins import zip
-from builtins import str
+#from builtins import str
 from builtins import range
 
 import itertools

nifty/spaces/power_space/power_space.py

@@ -16,7 +16,7 @@
 # NIFTy is being developed at the Max-Planck-Institut fuer Astrophysik
 # and financially supported by the Studienstiftung des deutschen Volkes.
 
-from builtins import str
+#from builtins import str
 import ast
 import numpy as np