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This is a known problem with Python baking the compiler used to compile it into the binary and using it to compile any python modules. You can fix this by either (1) rebuilding all your ports (the only way to be sure), or (2) by rebuilding your python: sudo port -n upgrade --force python27, sudo port clean mercurial and try again.
While you're at it, you might want to rebuild your perl installation, if you happen to have any, the same way.
