[MacPorts] #60419: clang-9.0 and dependents cannot be built on 10.6 because of a dependency cycle

[MacPorts]

#60419: clang-9.0 and dependents cannot be built on 10.6 because of a dependency
cycle
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  Reporter:  ryandesign                            |      Owner:  (none)
      Type:  defect                                |     Status:  new
  Priority:  Normal                                |  Milestone:
 Component:  ports                                 |    Version:  2.6.2
Resolution:                                        |   Keywords:
      Port:  clang-9.0 cmake curl libpsl python38  |
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Comment (by ryandesign):

 The python37 portfile uses `PortGroup clang_dependency 1.0` to combat
 this; the python38 portfile does not. Presumably it should be added to the
 python38 portfile. Any remaining clang dependencies that use python37
 should presumably be switched to use python38 and then the portgroup can
 be removed from python37.

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Ticket URL: <https://trac.macports.org/ticket/60419#comment:2>
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