Note that my main interest here is in making commands like:
py -3 -m pip install foo
cross platform rather than Windows specific. At the moment we don't have a way to explicitly invoke Python 3 that works everywhere.
It might be a lot simpler to get just that than the full py script:
If you just run python (off the path), unless -3 is specified, in which case you run python3 instead.
Doesn't that over all known distros, third-party installations, and pristine local installs of 3.x?
If you also want to make -2 portable, it seems like that requires some configuration (as long as some distros install 3.x as python, and others install 2.x as just python and not also python2). But is that a requirement!