cve-check: neaten get_cve_info

Remove obsolete Python 2 code, and use convenience methods for neatness.

(From OE-Core rev: f19253cc)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>