[MacPorts] #62174: cmake 3.19.3 tries to use sphinx-build-3.6

[MacPorts]

#62174: cmake 3.19.3 tries to use sphinx-build-3.6
-------------------------+--------------------
  Reporter:  EJFielding  |      Owner:  (none)
      Type:  defect      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.6.4
Resolution:              |   Keywords:
      Port:  cmake       |
-------------------------+--------------------

Comment (by EJFielding):

 At the beginning of the log file, it correctly finds `py37-sphinx`:
 ```
 :debug:main Searching for dependency: py37-sphinx
 :debug:main Found Dependency: receipt exists for py37-sphinx
 ```
 But later tries to use sphinx-build-3.6.

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Ticket URL: <https://trac.macports.org/ticket/62174#comment:1>
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