I concur with Terry.

Actually, registering the browser is not necessary for using it. Set the environment variable BROWSER and use your favorite browser. The following example uses curl (which is not registered as in the webbrowser module) as a browser:

BROWSER=curl python3 -m webbrowser -n http://example.org

The registry in the webbrowser module is used for the case if you use Python on some non-mainstream platform which does not have OS-level infrastructure for running default browser or settings are empty. I.e. it is not Windows or macOS and does not support OpenDesktop and Posix specifications, and BROWSER is not set. If you have such marginal system and use marginal browser on it, it is worth to spend some time for setting BROWSER or configuring www-browser, x-www-browser, xdg-open, etc.