Izar Tarandach is a threat modeling pioneer, seen as one of the movers and shakers in the threat modeling world. Izar leads a small team that develops the pytm tool, which is self-described as a "A Pythonic framework for threat modeling". The GitHub page goes on to say define your system in Python using the elements and properties described in the pytm framework. Based on your definition, pytm can generate, a Data Flow Diagram (DFD), a Sequence Diagram and most important of all, threats to your system.
Reach out to Izar on Twitter and visit the pytm GitHub page to download and try this tool out for yourself!