> #+NAME: test2
> #+begin_src python :results value :preamble # -*- coding: utf-8 -*- :return
> a
> a = ( ( "é", "a" ), ( "a", "à" ) )
> b = "é"
> #+end_src
>
> #+RESULTS: test2
> | \303\251 | a        |
> | a        | \303\240 |
>

Maybe this isn't an execution problem, but is rather a buffer encoding
problem.  I executed your example above in a small buffer (attached).  I
then saved this buffer and was forced to specify an encoding, I selected
utf8.  If I cat the resulting file from disk, the accented characters
appear correctly.