Skip test_find_mac on Windows (issue #19804).

This test requires the ifconfig executable on $PATH, /sbin/, or /usr/sbin.

Lib/test/test_uuid.py

@@ -347,6 +347,7 @@ class TestUUID(unittest.TestCase):
 
         self.assertEqual(node1, node2)
 
+    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
     def test_find_mac(self):
         data = '''\