Skip test_find_mac on Windows (issue #19804).

This test requires the ifconfig executable on $PATH, /sbin/, or /usr/sbin.
diff --git a/Lib/test/test_uuid.py b/Lib/test/test_uuid.py
index 55b939f..e623417 100644
--- a/Lib/test/test_uuid.py
+++ b/Lib/test/test_uuid.py
@@ -347,6 +347,7 @@
 
         self.assertEqual(node1, node2)
 
+    @unittest.skipUnless(os.name == 'posix', 'requires Posix')
     def test_find_mac(self):
         data = '''\