s4/selftest: enable samba4.schemaInfo.python for py3

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Sep  6 02:27:51 CEST 2018 on sn-devel-144
