luhn: Add test data version

exercises/luhn/luhn_test.py

@@ -5,6 +5,8 @@ import unittest
 from luhn import Luhn
 
 
+# test cases adapted from `x-common//canonical-data.json` @ version: 1.0.0
+
 class LuhnTests(unittest.TestCase):
     def test_single_digit_strings_can_not_be_valid(self):
         self.assertFalse(Luhn("1").is_valid())