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