We need additional python packages to run check-acceptance. Add them
to the docker images we will be using later.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200701135652.1366-26-alex.bennee@linaro.org>