README: update for Python 3

Convert the usage to be compatible with Python 3 and the current API.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Message-Id: <20190817212532.15661-2-luca@z3ntu.xyz>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
1 file changed