Comment 2 for bug 1809452

One possible cause might be the tornado package itself.

Upstream salt has the following dependency requirements.

---
tornado>=4.2.1,<6.0; python_version < '3'
tornado>=4.2.1,<5.0; python_version >= '3.4'
---

However the tornado package maintainer pushed the untested/unsupported version 5.0.2-1build1 into the cosmic release.