BUG: fix PY2 slice handling in Atom class

Override `__setslice__` from the base ndarray class.

Author: pavoljuhas

src/diffpy/structure/atom.py

@@ -18,6 +18,8 @@
 """
 
 import numpy
+import six
+
 from diffpy.structure.lattice import cartesian as cartesian_lattice
 
 # conversion constants
@@ -530,4 +532,14 @@ def __array_wrap__(self, out_arr, context=None):
         """
         return out_arr.view(numpy.ndarray)
 
+    # Python 2 Compatibility -------------------------------------------------
+
+    if six.PY2:
+
+        def __setslice__(self, lo, hi, sequence):
+            self.__setitem__(slice(lo, hi), sequence)
+            return
+
+    # ------------------------------------------------------------------------
+
 # End of _AtomCartesianCoordinates