lalinference: backport a6c8ae7f to release branch
commit a6c8ae7f32524aba5de7d5a33de5f4db7b03e74d
Author: Steffen Grunewald <steffen.grunewald@ligo.org>
Date: Thu Dec 12 12:20:17 2019 -0600
Activate python3 builds. Minor additions to description.