Fix compatibility with pygobject-3.50

[AngryLoki]

Gdk.RGBA constructor now ignores parameters:

>>> Gdk.RGBA(1,1,1,1)
<stdin-1>:1: DeprecationWarning: Passing arguments to gi.types.Boxed.__init__() is deprecated. All arguments passed will be ignored.

Note, that I only tested in a single setup, which looks like this:

• glib-2.84.4
• pygobject-3.50.1
• gtk-4.20.3
• libadwaita-1.8.2
• gtksourceview-5.16.0

Nevertheless, this change should be useful, if not now, then maybe later.

Closes #256