Wild .decode make some functions unusable in python3

[P-EB]

Hey,

In UnstructuredGrid.py, as for an example, there is a function unstructured_grid_fromfile, the first line calls a .decode() that won't work with Python3, since str are not decode-able.

The Python3 support seems not totally functionnal for now.

A patch would be to use unicode_literals from future in both python2 and 3