cve-update-nvd2-native: always pass str for json.loads()

cve-update-nvd2-native: always pass str for json.loads()

Currently json.loads() accepts one of the types str, bytes, or bytearray
as an argument, but bytes and bytearrays have only been allowed since
python 3.6. The version of Python3 provided by default on Ubuntu 16.04
and Debian 9.x is 3.5, so make raw_data type str to work correctly on
these build hosts.

Signed-off-by: Yuta Hayama <hayama@lineo.co.jp>
Signed-off-by: Steve Sakoman <steve@sakoman.com>