[MacPorts] #57927: waf PortGroup: make the python version configurable

[MacPorts]

#57927: waf PortGroup: make the python version configurable
-------------------------+--------------------
 Reporter:  yan12125     |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  mpv
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 Currently in `_resources/port1.0/group/waf-1.0.tcl`, Python 2.7 is hard-
 coded. It would be great to get the Python version configurable, so that
 ports using waf (e.g. mpv) can gradually move from Python 2.7 to Python
 3.x.

 For now the mpv port still uses Python 2.7 to build even with variant
 +python37. See https://github.com/macports/macports-ports/pull/3431 for
 some related discussions.

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Ticket URL: <https://trac.macports.org/ticket/57927>
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