Bump build-time system python's requirement to 2.6.

"Writing Python code that is OK on 2.x and 3.x is next to impossible
when 2.x is 2.5" --Miklos Vajna

Change-Id: I432c2773eb60ab3bd5cc907407ca5159ce4390ad
Reviewed-on: https://gerrit.libreoffice.org/10116
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>