port selecting python3 as python should emit a warning

[jeremyhu (Jeremy Huddleston Sequoia)]

Using any version of python 3 as the python interpreter is not supported. python should only refer to python2. If users want to try configuring their system in this unsupported manner, they should be warned by 'port select' when doing so.

See ​https://www.python.org/dev/peps/pep-0394

[ryandesign (Ryan Carsten Schmidt)]

We should just make port select python only able to select python 2.x versions. It would fix problems like #32575 and #49591. Users should use port select python3 to set the python3 symlink. We could even consider deprecating the python2_select port and rolling its functionality (i.e. setting the python2 symlink) into the python_select port (or vice versa).

[raimue (Rainer Müller)]

Moving this to the ports that need to implement ​PEP 394.

[yan12125 (Chih-Hsuan Yen)]

I don't think there is a need for such a change now. Some reasons:

1. PEP 394 has been updated to allow pointing "python" to a python 3.x interpreter (1).
2. Python 2 is going to be dead. It's natural for users to select python 3.x as the default python command.
3. Issues like #32575 and #49591 should be fixed in individual ports. For example, patch their build system to use python 2.x instead merely "python".

(1) ​https://github.com/python/peps/commit/ae932bd6fd2c493d7d64ce328a5293bac4c48884