pyldb: Fix reference leaks

pyldb: Fix reference leaks

The parse_ldif and MessageElement.__iter__ functions leaked references
to intermediate lists whose iterators they return.

The MessageElement repr used the undocumented macro PyObject_REPR, which
leaks references. (It was used internally in CPython before fatal errors,
and will be removed in Python 3.5.)

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Jelmer Vernooij <jelmer@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>