*-3.*: Switch pygobject deps to pygobject-python2

The previous commit switched pygobject to Python 3 by default. As our current
modules expect Python 2, switch all dependencies to pygobject-python2 for now.
As modules get ported to Python 3, they should switch their dependency back to
pygobject.

https://bugzilla.gnome.org/show_bug.cgi?id=687743