[MacPorts] #61338: asciinema @2.0.2: update Python to 3.8

[MacPorts]

#61338: asciinema @2.0.2: update Python to 3.8
------------------------+-----------------------
 Reporter:  FranklinYu  |      Owner:  larryv
     Type:  update      |     Status:  assigned
 Priority:  Normal      |  Milestone:
Component:  ports       |    Version:
 Keywords:              |       Port:  asciinema
------------------------+-----------------------
 Currently this port depends on `py37-setuptools`, while all my other
 Python tools have been using Python 3.8.

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Ticket URL: <https://trac.macports.org/ticket/61338>
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