Try using np.unicode_ dtype for Python 3 compatibility

See https://docs.scipy.org/doc/numpy/reference/arrays.dtypes.html?highlight=dtype%20unicode#string-dtype-note

src/omero/columns.py

@@ -266,7 +266,7 @@ def dtypes(self):
         (Testing suggests this may not be necessary, the size appears to be
         correctly set at initialisation)
         """
-        return [(self.name, "S", self.size)]
+        return [(self.name, "U", self.size)]
 
     def descriptor(self, pos):
         # During initialization, size might be zero