python: Make the code compatibly with 3.2

The previous change used some 3.3-specific functions.
We still support 3.2 so revert to 3.2-compatible calls.

Reported-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>