dist: use macosx_10_9_x86_64 by default for Python 3.8+

This is because Python 3.8 is only available for macOS 10.9+, and no longer for 10.6.

direct/src/dist/commands.py

@@ -240,6 +240,9 @@ class build_apps(setuptools.Command):
             'macosx_10_6_x86_64',
             'win_amd64',
         ]
+        if sys.version_info >= (3, 8):
+            # This version of Python is only available for 10.9+.
+            self.platforms[1] = 'macosx_10_9_x86_64'
         self.plugins = []
         self.embed_prc_data = True
         self.extra_prc_files = []