Closes #15620: check for presence of readline.clear_history(), which is...

Closes #15620: check for presence of readline.clear_history(), which is apparently missing on some readline versions, before calling it in the test.

Lib/test/test_readline.py

@@ -17,7 +17,9 @@ class TestHistoryManipulation (unittest.TestCase):
                      "The history update test cannot be run because the "
                      "clear_history method is not available.")
     def testHistoryUpdates(self):
-        readline.clear_history()
+        # Some GNU versions of readline do not support clear_history
+        if hasattr('readline', 'clear_history'):
+            readline.clear_history()
 
         readline.add_history("first line")
         readline.add_history("second line")