Look at diagrams.net (formerly draw.io). It is gratis (free of cost). I use it intensively as a software engineer to architect and design different workflows.
There is also a desktop app for the same web tool.
Before you try MS Visio, have a look at yEd. It's free, works on Windows as well and can also draw flowcharts.
You can adjust the colors etc. to make it look closer to Visio.
Downside: I needed to get used to its usability. It doesn't behave like I expected it would. Usually I accidentally draw a lot of more boxes than needed.
You can try pyreverse. It's installed as part of the pylint package so it's possible you already have it. Someone else already beat me to a pyreverse explainer: https://stackoverflow.com/questions/36432630/class-diagram-viewer-application-for-python3-source
Diagram is a feature-rich react component for visualizing, creating, and editing interactive diagrams like flow charts, organizational charts, mind maps, BPMN diagrams, and more. Its rich feature set includes built-in shapes, editing, serializing, exporting, printing, overview, data binding, and automatic layouts.
Herewith we have a created a simple sample ...
You might want to look at https://github.com/pierre3/PlantUmlClassDiagramGenerator It can be run standalone. It looks at your source file and generates rudimentary class diagrams.
Might be worth a look.