Why do you think this isn't a good idea?

As a user, what am supposed to do with that warning:
* about a program I have no idea I was calling (the webbrowser module abstraction is here so I don't have to think about it)
* I have no control about it's invocation
* the output comes asynchronously in my program and may be hard to link the the webbrowser.open call

Also the specific example of this issue is a xdg-open deprecation warning, but it could be any other output that is hard for the user to take action on or understand where it's coming from.

My understanding of the *current* code is that  all graphical browsers are already started with their output tossed away (https://hg.python.org/cpython/file/tip/Lib/webbrowser.py#l196). It seems weird to keep it only for all the command line wrappers (xdg-open, gnome-open, etc.).
Of course the TTY interactive browsers have a good reason to keep it, obviously.