[MacPorts] #62212: spice-gtk: Meson build fails with "Python module six not found" with Python 3.9

[MacPorts]

#62212: spice-gtk: Meson build fails with "Python module six not found" with Python
3.9
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  Reporter:  rseichter        |      Owner:  danchr
      Type:  defect           |     Status:  assigned
  Priority:  Normal           |  Milestone:
 Component:  ports            |    Version:  2.6.4
Resolution:                   |   Keywords:
      Port:  spice-gtk meson  |
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Comment (by frank-f):

 I am seeing the same error ever since I updated to Big Sur, which was a
 few months ago already. I had somehow managed to install everything into
 one python version before, but I am clueless what is causing this error.
 Any idea what changed in Big Sur that broke GtkSource.View()?

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Ticket URL: <https://trac.macports.org/ticket/62212#comment:11>
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