How to get total screen size in Python / GTK without using deprecated Gdk.Screen.get_width()?

I want the X/Y pixel dimensions of the entire desktop (potentially spanning multiple monitors), e.g. to determine the optimal resolution of a background image.

This Python code still works:

from gi import require_version
require_version("Gdk", "3.0")
from gi.repository import Gdk
screen = Gdk.Screen.get_default()
print(screen.get_width(), " ", screen.get_height())

but prints a deprecation warning:

<string>:7: DeprecationWarning: Gdk.Screen.get_width is deprecated
<string>:7: DeprecationWarning: Gdk.Screen.get_height is deprecated

The Gdk.Screen API docs just note:

Deprecated since version 3.22: Use per-monitor information instead

Other answers (like How do I get monitor resolution in Python or How to detect a computer's physical screen size in GTK) mention lots of other APIs and toolkits, or just give per-monitor information (like this for PyGtk), but I think it should still be possible (also in the future) to get the dimensions of the entire desktop via PyGtk. (After all, the deprecated function also still provides this.)

from How to get total screen size in Python / GTK without using deprecated Gdk.Screen.get_width()?