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.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/3491
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>