Python - gtk3 add stock icons to Gtk.Buttons

Try:

image = Gtk.Image(stock=Gtk.STOCK_OPEN)
self.browse_button = Gtk.Button(label="Some Label", image=image)

See the documentation.

Like Ptomato did, but with no label and using one line :

self.btnOpen = Gtk.Button(None,image=Gtk.Image(stock=Gtk.STOCK_OPEN))