python-pygobject: use Python 2 instead of Python 3

python-pygobject: use Python 2 instead of Python 3

Ideally this would be an option but that means conditional inherits.  For now
Python 2 is clearly the default Python in oe-core so switch to Python 2 instead
of 3.

This stops python3 being pulled inadvertently into builds, and appearing in
images that have ptest-pkgs installed (via dbus-test).

(From OE-Core rev: 7ae7277a577b2732eff7812860a7c7967644e532)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>