[MacPorts] #63069: vtk: migrate from python variants to subports

[MacPorts]

#63069: vtk: migrate from python variants to subports
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  Reporter:  mascguy      |      Owner:  stromnov
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:  2.7.2
Resolution:               |   Keywords:
      Port:  vtk          |
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Comment (by Dave-Allured):

 @mascguy Thanks for working on this `vtk` subport migration, and for
 explaining the details.  Please finish it if you are so inclined.  I do no
 yet have sufficient understanding of subport details.  Let me know if I
 may be of any help.

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Ticket URL: <https://trac.macports.org/ticket/63069#comment:16>
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