[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:  michaelld
      Type:  defect      |     Status:  assigned
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:  2.6.4
Resolution:              |   Keywords:
      Port:  cmake       |
-------------------------+-----------------------

Comment (by blair):

 I have +doc and +python38 in `/opt/local/etc/macports/variants.conf`.

 I ran into the same issue. If I revert cmake back to `906a898f14b` which
 is `cmake: update to 3.19.3` then the build works. Then going to
 `7caeafd874e` the build fails.

 Doing a diff between the last good build at `906a898f14b` and
 `7caeafd874e` shows this:

 {{{
  --->  Patching macports.cmake:
 s|__APPLICATIONS_DIR__|/Applications/MacPorts|g
 ---->  Patching macports.cmake: s|__PYTHON_VERSION_WITH_DOT__|3.8|g
 +--->  Patching macports.cmake: s|__PYTHON_VERSION_WITH_DOT__|3.7|g
  --->  Configuring cmake
 }}}

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Ticket URL: <https://trac.macports.org/ticket/62174#comment:4>
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