pidl:Python: make use of the pyrpc_{import,export}_union() functions

pidl:Python: make use of the pyrpc_{import,export}_union() functions

Pick up a change from Samba:

    commit f7707c04a5e039e757b288ad67a0c3cac9c6466a
    Author: Stefan Metzmacher <metze@samba.org>
    Date:   Mon Sep 12 16:33:38 2016 +0200

        pidl:Python: make use of the pyrpc_{import,export}_union() functions

        This avoids the linking problems at C shared object level.

        Signed-off-by: Stefan Metzmacher <metze@samba.org>
        Reviewed-by: Andreas Schneider <asn@samba.org>
Change-Id: I62c1d03b10e965552a4d598fa2370692ee0cd12a
Reviewed-on: https://code.wireshark.org/review/23713
Reviewed-by: Guy Harris <guy@alum.mit.edu>