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

diff --git a/Lib/http/__init__.py b/Lib/http/__init__.py
index e14a1eb..350afe7 100644
--- a/Lib/http/__init__.py
+++ b/Lib/http/__init__.py
@@ -15,6 +15,7 @@
         * RFC 7238: Permanent Redirect
         * RFC 2295: Transparent Content Negotiation in HTTP
         * RFC 2774: An HTTP Extension Framework
+        * RFC 7725: An HTTP Status Code to Report Legal Obstacles
         * RFC 7540: Hypertext Transfer Protocol Version 2 (HTTP/2)
     """
     def __new__(cls, value, phrase, description=''):
@@ -114,6 +115,10 @@
         'Request Header Fields Too Large',
         'The server is unwilling to process the request because its header '
         'fields are too large')
+    UNAVAILABLE_FOR_LEGAL_REASONS = (451,
+        'Unavailable For Legal Reasons',
+        'The server is denying access to the '
+        'resource as a consequence of a legal demand')
 
     # server errors
     INTERNAL_SERVER_ERROR = (500, 'Internal Server Error',
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',