The core packaging code now supports building Python 3 installers
using PyOxidizer. Let's teach the automation code to invoke it so
that we produce both Python 2 and Python 3 based exe installers.
When publishing the artifacts, the Python 3 versions are preferred
over the Python 2 versions given their higher weight (10 versus 9).
This may be a controversial change. But I think making Python 3
the default is warranted, as it is the future. The Python 2 installers
are still fully supported and can be installed should issues with
Python 3 arise.