Support python27 pickle by converting str to bytes. (#88)

src/google/appengine/ext/ndb/model.py

@@ -3043,7 +3043,16 @@ def __get_arg(cls, kwds, kwd):
   def __getstate__(self):
     return self._to_pb().SerializeToString()
 
+  def _py2_compat_encode(self, py2_serialized_pb):
+    return bytes(py2_serialized_pb, 'latin1')
+
   def __setstate__(self, serialized_pb):
+    if isinstance(serialized_pb, str):
+      # If 'serialized_pb' was written by a python27 clone.
+      logging.warning(
+          'Assuming python2 pickled state, converting to python3 type.'
+      )
+      serialized_pb = self._py2_compat_encode(serialized_pb)
     pb = entity_pb2.EntityProto.FromString(serialized_pb)
     self.__init__()
     self.__class__._from_pb(pb, set_key=False, ent=self)