[MacPorts] #63683: py-altair: add python39?

[MacPorts]

#63683: py-altair: add python39?
--------------------+-----------------------
 Reporter:  jowens  |      Owner:  stromnov
     Type:  update  |     Status:  assigned
 Priority:  Normal  |  Milestone:
Component:  ports   |    Version:
 Keywords:          |       Port:  py-altair
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 Please consider adding python39 to the list of supported pythons in py-
 altair (if it's supported, anyway).

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Ticket URL: <https://trac.macports.org/ticket/63683>
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