Fix support for fundamental (primitive) types, such as Gst.Bitmask

Fix regression causing fundamental primitives (not backed by instance/class data objects).

I created a replica of Gst.Bitmask and used that for testing.

@jbicha Can you please verify this fixes the issues with Gst for you?

Fixes #624 (closed)

NB. I branched PyGObject 3.48, not master.