[MacPorts] #67116: boost Python variants should be subports for automated installs with CI and buildbots

[MacPorts]

#67116: boost Python variants should be subports for automated installs with CI and
buildbots
-----------------------+---------------------------------------
 Reporter:  essandess  |      Owner:  (none)
     Type:  defect     |     Status:  new
 Priority:  Normal     |  Milestone:
Component:  ports      |    Version:
 Keywords:             |       Port:  boost, boost171, boost176
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 The Python version variants of boost are implemented as variants, not
 subports. This breaks CI and buildbot builds pf ports that depend on boost
 being installed with a specific Python version, e.g., {{{py-graph-tool}}},
 https://github.com/macports/macports-
 ports/pull/17914#issuecomment-1474822953

 Additionally, the correct way to implement such dependencies is via
 subports, https://trac.macports.org/ticket/126#comment:48:
     "The progress that has been made is that MacPorts now supports
 subports, which make it easier for a single portfile to provide multiple
 separately-installable ports. Ports that provide optional functionality
 that other ports might need to depend on should break that optional
 functionality out into subports instead of using variants. We should not
 pursue the idea of allowing variants to be depended upon."

 These variants should be converted to subports like
 {{{boost176-python311}}} etc., which could have the added benefit of
 avoiding current conflicts between these python port variants.

-- 
Ticket URL: <https://trac.macports.org/ticket/67116>
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