Having python3 actually pointing to python2 is bad. So we stop doing so.
In addition we need to re-introduce a python executable since some of the
script really need to be able to say "current python" in their shbang. For
example, hghave is one of such script.
The faulty changes where introduced by c102b704edb5.
chmod +x will need to be gated by a #if execbit on Windows, but let's figure it out on a Windows machine and take this as is for now as there might be an issue with the script running at all without an sh prefix.