How do I paint an always-on-top green circle on my macOS display?

How do I create a program that paints an always-on-top small green circle on my screen?

Is it possible in AppleScript, Python or Java? Is Swift or Objective-C needed? Solutions in all programming languages are most welcome!

It was simple to create a macOS menu bar app using Python, Rumps, and py2app.

This guide explains the basics, which is basically:

import rumps

class GreenCircleApp(object):
    def __init__(self):
        self.app = rumps.App("GreenCircle", "🟢")

    def run(self):
        self.app.run()

if __name__ == '__main__':
    app = GreenCircleApp()
    app.run()

The circle can easily be changed using e.g. self.app.title = "🔴"