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

[MacPorts]

#43733: openslide: use "github.tarball_from releases"
--------------------------+------------------------
 Reporter:  ryandesign@…  |      Owner:  bgilbert@…
     Type:  enhancement   |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.2.99
 Keywords:  haspatch      |       Port:  openslide
--------------------------+------------------------
 This patch makes openslide use "github.tarball_from releases" (which was
 just added to the github portgroup) instead of manually setting
 master_sites.

 If you have no objections I'd be happy to commit it.

-- 
Ticket URL: <https://trac.macports.org/ticket/43733>
MacPorts <http://www.macports.org/>
Ports system for OS X