[MacPorts] #68822: py-chainer: update to current version

[MacPorts]

#68822: py-chainer: update to current version
--------------------------+------------------------
 Reporter:  barracuda156  |      Owner:  (none)
     Type:  update        |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.8.1
 Keywords:                |       Port:  py-chainer
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 The port version is archaic, current one is 7.8.1:
 https://github.com/chainer/chainer/tags

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Ticket URL: <https://trac.macports.org/ticket/68822>
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