Add Python 3 compatibility to GDB pretty printers.

GDB on *buntu is linked against Python 3.3, which has many
incompatibilities to Python 2, resulting in broken code.

This patch uses the Python six library as a compatibility layer.

Change-Id: Icb4cc54a1d05afb119376bb5e1430c91cb794d08
Reviewed-on: https://gerrit.libreoffice.org/6688
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>