Buttons is Gtk.Dialog can't be separated?

[John Coppens <john jcoppens com>]

Hi all.

I mad a small function using Gtk.MessageDialog, and add the requisite
Ok and Cancel buttons. But, on the result shown the buttons are glued
together and expand to occupy the entire width of the dialog. Using
style_get_property, I get reply '4', but this spacing is not visible.
Also, probably generating the 4 just mentioned, I found a parameters in
the CSS file defining the spacing at 4.

get_action_area() is flagged as deprecated but does work - I can set
the button spacing as wanted.

In short: How can I set the button spacing (and other props) if:
        - apparently the CSS spacing isn't taken into account
        - the button_spacing style property is set to 4 but not used
        - I shouldn't use get_action_area() to set the spacing?

I'm using Gtk 3.22 at the moment, and via introspection, Python 3.6. To
be sure this wasn't a result of the loaded theme, I also tried with
Adwaita and others.

Attached are a small test program, and a screenshot of a dialog.