[MacPorts] #57767: py36-graph-tool uses incorrect python version 2.7

[MacPorts]

#57767: py36-graph-tool uses incorrect python version 2.7
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  Reporter:  ssllmit              |      Owner:  count0
      Type:  defect               |     Status:  assigned
  Priority:  Normal               |  Milestone:
 Component:  ports                |    Version:  2.5.4
Resolution:                       |   Keywords:
      Port:  py-graph-tool boost  |
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Comment (by mf2k):

 I normally have my preferred python added as a default variant in my
 variants.conf file to keep all of my ports synced on the same python
 version. However, I think I had to remove it one time because of problems
 building boost with python36. So I think changing boost to default to
 python36, or better yet python37, makes sense so that we know it works.

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Ticket URL: <https://trac.macports.org/ticket/57767#comment:7>
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