'PyUnicode_GetSize' is deprecated [-Werror,-Wdeprecated-declarations]

Change-Id: Id298d6a2b0af9ce418f9124c9addda55fcdc0318
Reviewed-on: https://gerrit.libreoffice.org/62687
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>