Build by default for python 3
Build by default for python 3

5fa33b2 introduced build for either python 2 or python 3, with the
default of python 2.

`configure` and `autogen.sh` now accept `--with-python3` or
`--with-python2` options, with the default of python 3.

Signed-off-by: James Cameron <979be511e3d216882811a349af33a6d24f3f5f99@laptop.org>

sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3,sugarlabs/sugar-toolkit-gtk3