The intended future for the ``python`` command is:

> ``python`` doesn't exist and one always has to specify ``python2``
> or ``python3``.

( – Guido, python#630 (comment) )

There are three conflicting ideas around the ``python`` command,
longer-term:

1. The ``python`` command should continue to refer to Python 2 (if
   Python 2 is available).
2. ``python`` is a correct shebang for py2/py3 source-compatible scripts.
3. Python 2 should *not* be available (by default / after 2010).

One of these has to give.
It seems that (2) is the easiest to shed, so this proposal does just that.

* Make ``python3`` the preferred shebang for py2/py3 compatible
  scripts, as that's the only shebang that'll work on py2-less systems.
  (An exception is made for scripts targetting the *system* Python
  on e.g. macOS/RHEL.)
* Make the unversioned ``python`` command optional (along with
  ``idle``, ``pydoc``, and ``python-config``).
  Distributions now do not need to install it by default, even if
  Python 2 is installed. (But they should make it installable
  explicitly, as long as they ship Python 2.)
  This should introduce more people to systems without the
  "legacy" ``python`` command, encourage the use of explicit
  ``python2``/``python3``, and ease switching to systems that don't
  have Python 2 at all.
* Clarify that distributions *should not* make unversioned
  ``python`` configurable. (That might work for carefully managed
  systems, but the ecosystem should converge on ``python3``, not
  "your ``python`` needs to be set properly".)
* Remove mentions of choosing to link ``python`` to ``python3`` in the
  future, as we don't expect to start recommending that.

* Use the term **"unversioned"** ``python`` when contrasting it with
  ``python3``/``python2``.
  I found that this makes the message much clearer.