Gaphor (https://github.com/gaphor/gaphor) is an open source UML tool written in Python. The goal is to create a simple, easy to use modeling tool not a big enterprise tool like the ones already mentioned. I would like to soon support SysML which is for modeling systems design and requirements. We are finishing up converting it to Python3 and Gtk+3 now. We would love more involvement or input if there are things you would like to see supported.