I have installed Python3.7.2 and PyGObject GTK3 on MacOS 10.13.6, both with Homebrew.
Running the test gtk3-demo I can see there are images missing, and there are warnings:
(gtk3-demo:6501): Gtk-WARNING **: 11:33:01.313: Could not load a pixbuf from /org/gtk/libgtk/theme/Adwaita/assets/bullet-symbolic.svg. This may indicate that pixbuf loaders or the mime database could not be found.
As there is no /org directory installed, it looks like the entire images library is missing. This github comment from June 2017 seems relevant, but is not a solution.
Any suggestions on how to fix this please?