[MacPorts] #43733: openslide: use "github.tarball_from releases"

[MacPorts]

#43733: openslide: use "github.tarball_from releases"
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  Reporter:  ryandesign@…  |      Owner:  bgilbert@…
      Type:  enhancement   |     Status:  new
  Priority:  Normal        |  Milestone:
 Component:  ports         |    Version:  2.2.99
Resolution:                |   Keywords:  haspatch
      Port:  openslide     |
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Comment (by bgilbert@…):

 Looks good to me.  Thanks.

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Ticket URL: <https://trac.macports.org/ticket/43733#comment:1>
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