fix LightProof measurement conversion for Python 3

Change-Id: I6d36bc804271956e1c8649df0832615311fcf5e1
Reviewed-on: https://gerrit.libreoffice.org/#/c/9367
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>