py3: fix regression in schemaupdate
    
    The python-ldap classes that process schema definitions require a
    unicode string, not a byte string.  A recent py3 compatibility fix
    (d89de4219d0e8ee33e81d6b6d1bc6c22ac9ffbaa) changed the constructor
    argument to a unicode string to dispel a warning, but this broke
    schema update.  Change it back to a bytestring.
    
    Part of: https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>