[MacPorts] #63239: Bug in mpv portfile

[MacPorts]

#63239: Bug in mpv portfile
---------------------------+--------------------
 Reporter:  gaming-hacker  |      Owner:  (none)
     Type:  defect         |     Status:  new
 Priority:  Normal         |  Milestone:
Component:  ports          |    Version:  2.7.1
 Keywords:  bug            |       Port:  mpv
---------------------------+--------------------
 Is
 ```
 waf.python.branch ${python.branch}
 ```

 should be
 ```
 set waf.python.branch ${python.branch}
 ```

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Ticket URL: <https://trac.macports.org/ticket/63239>
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