bpo-39507: Add HTTP status 418 "I'm a Teapot" (GH-18291)

diff --git a/Lib/test/test_httplib.py b/Lib/test/test_httplib.py
index 7f4decc..77d4335 100644
--- a/Lib/test/test_httplib.py
+++ b/Lib/test/test_httplib.py
@@ -1433,6 +1433,7 @@
             'UNSUPPORTED_MEDIA_TYPE',
             'REQUESTED_RANGE_NOT_SATISFIABLE',
             'EXPECTATION_FAILED',
+            'IM_A_TEAPOT',
             'MISDIRECTED_REQUEST',
             'UNPROCESSABLE_ENTITY',
             'LOCKED',