bpo-29854: Skip history-size test on older readline (GH-2621)

Turns out that history-size was added in readline 6.0. This explain why
this tests fail on FreeBSD when using readline 5.2. We skip now the
history size if readline does not support it.

See https://cnswww.cns.cwru.edu/php/chet/readline/CHANGES for
details.

Lib/test/test_readline.py

@@ -226,6 +226,13 @@ print("history", ascii(readline.get_history_item(1)))
         self.assertIn(b"result " + expected + b"\r\n", output)
         self.assertIn(b"history " + expected + b"\r\n", output)
 
+    # We have 2 reasons to skip this test:
+    # - readline: history size was added in 6.0
+    #   See https://cnswww.cns.cwru.edu/php/chet/readline/CHANGES
+    # - editline: history size is broken on OS X 10.11.6.
+    #   Newer versions were not tested yet.
+    @unittest.skipIf(readline._READLINE_VERSION < 0x600,
+                     "this readline version does not support history-size")
     @unittest.skipIf(is_editline,
                      "editline history size configuration is broken")
     def test_history_size(self):