python: Do not mix bytes and unicode strings

python: Do not mix bytes and unicode strings

Mixing the two is a long-standing recipe for errors in Python 2, so much
so that Python 3 now completely separates them.

This commit stops treating both as if they were the same, and in the
process makes the script compatible with both Python 2 and 3.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>