PYTHON: Define constants as bytes instead of strings
When using python3 getsidbyname() and getnamebysid() expect the key as
bytes instead of strings, and currently those are defined as strings.
So, in order to avoid people working around this by doing
`pysss_nss_idmap.SID_KEY.encode('utf-8')` let's make their life easier
and properly have those constants defined as bytes.
Signed-off-by: Fabiano Fidêncio <email@example.com>
Reviewed-by: Michal Židek <firstname.lastname@example.org>
Reviewed-by: Jakub Hrozek <email@example.com>