pyldb: Fixes and Python3 compat for Dn component accessors

pyldb: Fixes and Python3 compat for Dn component accessors

Use "s#"/"z#" argument specifiers in set_component and
set_extended_component instead of converting strings manually.
(Under Python 3, This means both text strings and bytes are accepted.)

Raise error on set_component(None), instead of crashing.

Return text strings from get_{extended}_component under Python 3.

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>