[MacPorts] #57770: py-kaitaistruct: add py37 subport

[MacPorts]

#57770: py-kaitaistruct: add py37 subport
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 Reporter:  petrrr       |      Owner:  yan12125
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  py-kaitaistruct
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 Required by a one of the ports I maintain.

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Ticket URL: <https://trac.macports.org/ticket/57770>
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