Python3 should be default

@andyli waneck and @Simn

I ran into some awkwardness running travis CI for the Haxe python target. Haxe python doesn’t support python 2, (and shouldn’t, because python2 is dead). But on the instances used by the haxe CI configuration, python resolves to python 2, so executing my tests usually fails when I use the same hxml file in the CI as on my local machine.

works locally:

build-scripts/all-script-args.hxml
build-scripts/test/all-test-args.hxml
--python bin/py/test-hiss.py
--cmd python bin/py/test-hiss.py

works in Travis:

build-scripts/all-script-args.hxml
build-scripts/test/all-test-args.hxml
--python bin/py/test-hiss.py
--cmd python3 bin/py/test-hiss.py

Since Python 3 is now the only supported version of python, I think it would be a little more ergonomic if the Haxe travis configuration provided python3 as python in PATH.

In dist: focal. Python 3 is the only available version. See https://github.com/travis-ci/travis-cookbooks/pull/1076#issuecomment-617480688 why Python 2 is the default in the previous versions.

If Haxe is anything worth its salt, there must be a way to specify the Python executable for it to use.