[pydry]

UML's problem is not really that it's too formal but that it's usually too detailed, too vague or too out of sync with the real system to be useful.

It was also heavily associated with Big Design Up Front, which has been more or less trashed in favor of agile / iterative improvements / spikes / whatever you want to call it.

PlantUML still seems to be a thing, although perhaps more for general diagramming.