Use ExtraAssertions for assertEndsWith(), added to TestCase in 3.14

Author: serhiy-storchaka

Lib/test/test_support.py

@@ -968,7 +968,7 @@ def test_skipped_without_subprocess_support(self):
         self.assertEqual(calls, [])
 
 
-class TestSubTests(unittest.TestCase):
+class TestSubTests(ExtraAssertions, unittest.TestCase):
 
     def run_test(self, cls):
         result = unittest.TestResult()