csrgen: accept public key info as Bytes
    
    cert_get_requestdata() method is meant for internal use only and
    is never passed a file. Make its parameter public_key_info Bytes
    to better represent what's actually being passed to it.
    
    https://pagure.io/freeipa/issue/7131
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>