Besides locking the dependency down to ~sci-libs/fcl-0.5.0 should be avoided if at all possible to fix it, sci-libs/fcl-0.5.0 is still EAPI-5. Upstream have a merge/fcl6 branch: https://github.com/BerkeleyAutomation/python-fcl/commits/merge/fcl6 AUR seem to be patching theirs with changes from https://github.com/CyrilWaechter/python-fcl/commits/master