[MacPorts] #29821: mercurial: allow using python27

[MacPorts]

#29821: mercurial: allow using python27
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 Reporter:  jowens@…             |       Owner:  deric@…           
     Type:  enhancement          |      Status:  new               
 Priority:  Normal               |   Milestone:                    
Component:  ports                |     Version:                    
 Keywords:                       |        Port:  mercurial         
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Comment(by macports@…):

 Oops, forgot to search for existing when filing multiple tickets and made
 a dupe #30255

 Since that is closed, attaching the patch from it here in hopes its
 useful. Its trivial, but maybe someone can just commit it and we can move
 on.

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Ticket URL: <https://trac.macports.org/ticket/29821#comment:2>
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