Rather than looking for pygobject-3.0 with pkg-config, run an actual
test in python, loading the 'gi' module.
I also added a version parameter making it possible to even check for a
required version of PyGObject, though it is currently unused (as I have
no idea if we have a minimum requirement for our plug-ins to work well).

This is an improvement of commit 8e938e09 as I realized that we don't
need any development files, therefore I don't think that searching with
pkg-config was the right idea in our case.