[MacPorts] #51572: port selecting python3 as python should emit a warning

[MacPorts]

#51572: port selecting python3 as python should emit a warning
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  Reporter:  jeremyhu@…   |      Owner:  macports-tickets@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:  2.3.4
Resolution:               |   Keywords:
      Port:               |
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Comment (by ryandesign@…):

 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).

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Ticket URL: <https://trac.macports.org/ticket/51572#comment:2>
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