Python 3.6.1 (default, Mar 27 2017, 00:27:06) 
[GCC 6.3.1 20170306] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import webbrowser
>>> webbrowser.open("https://www.google.com")
True
>>> This tool has been deprecated, use 'gio open' instead.
See 'gio help open' for more info.

A quick test reveals it is the invocation of xdg-open that outputs the warning:

$ xdg-open --version
xdg-open 1.1.0 rc3
$ xdg-open https://www.google.com
This tool has been deprecated, use 'gio open' instead.
See 'gio help open' for more info.

It can be quite annoying because in my program the message is outputted in a ncurses UI and it messes the display.

Possible changes to fix this:
* Silence xdg-open output (pass stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL in Popen call)
* Detect and use "gio open" if available