support/scripts/pkg-stats: decode subprocess output for python3

In Python 3, the functions from the subprocess module return bytes
(and no longer strings as in Python 2), which must be decoded for
further text operations.

Now, pkg-stats can be run in Python 3.

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>