run-tests: stop writing a `python3` symlink pointing to python2 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. Differential Revision: https://phab.mercurial-scm.org/D10943