Switch pygobject to python3, add pygobject-python2 project

According to https://live.gnome.org/GnomeGoals/Python3Porting and
https://mail.gnome.org/archives/desktop-devel-list/2012-November/msg00005.html
pygobject should now default to building for Python 3. Ensure that it does so
by explicitly configuring for --with-python=python3, and introduce a fake
module pygobject-python2 which builds pygobject for Python 2.

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