Why does this bug still exist? I was able to build headless Freeorion on Devuan with Python 3 just by changing the control file (see https://framagit.org/specing/debian-freeorion/-/commit/948beb224601838163bf7c9bea4d8bcf677dcb8d). It seems that Freeorion migrated from Python 2 a long time ago?
