Add python2/3 compatibility code for buffer

Web/Python/vtk/web/__init__.py

@@ -5,6 +5,8 @@ py3 = sys.version_info.major > 2
 if py3:
     def iteritems(d, **kwargs):
         return iter(d.items(**kwargs))
+    buffer = memoryview
 else:
     def iteritems(d, **kwargs):
         return d.iteritems(**kwargs)
+    buffer = buffer

Web/Python/vtk/web/dataset_builder.py

 from vtk import *
 from vtk.web.camera import *
 from vtk.web.query_data_model import *
-from vtk.web import iteritems
+from vtk.web import iteritems, buffer
 import json, os, math, gzip, shutil
 
 # Global helper variables