bpo-26589: Add http status code 451 (GH-15413)

diff --git a/Lib/test/test_httplib.py b/Lib/test/test_httplib.py
index 656932f..1121224 100644
--- a/Lib/test/test_httplib.py
+++ b/Lib/test/test_httplib.py
@@ -1401,6 +1401,7 @@
             'PRECONDITION_REQUIRED',
             'TOO_MANY_REQUESTS',
             'REQUEST_HEADER_FIELDS_TOO_LARGE',
+            'UNAVAILABLE_FOR_LEGAL_REASONS',
             'INTERNAL_SERVER_ERROR',
             'NOT_IMPLEMENTED',
             'BAD_GATEWAY',