same motivation as in #5816:
- distributions have scripts to rewrite shebangs on installation and
they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
there PATH correctly.
This is useful on systems like NixOS, where python3 is not in
/usr/bin/python3 as well as for people using alternative ways to
install python such as virtualenv/pyenv.