python2/3 in casa5/6

[teuben]

We use a few cases of #! /usr/bin/env python
which works in casa5, but not in casa6, because it only has python3.

Possible solution is to completely abandon casa5/python and fully switch to casa5/python3.

Also possible is to have our configure patch the offending files (currently only listfitsa)

What also works is patching the casa6 distro:

          cd $CASA_ROOT/lib/py/bin/;    ln -s python3 python

and if that's not allowed....

          cd $ADMIT/bin;   ln -s $CASA_ROOT/lib/py/bin/python3 python

which is probably the preferred solution, via configure.

[mpound]

I think abandoning explicitly using casa5/python3 is the most transparent solution. ln -s magic is hidden from user and potentially confusing.

[teuben]

if you look at CASA_ROOT/bin you will see them totally abusing ln.

I can't abandon casa5 yet until we have all python2->3 bugs resolved. So I'm inclined to use a symlink in ADMIT/bin, which will not affect non-ADMIT users, plus needs no write permission in casa's tree. Once we have a fully robust admit w/ casa6, we can save the old admit w/ casa5/python2/3 on a branch never to be touched.