[issue30553] Add HTTP Response code 421

[Julien]

New submission from Julien:

The `http.HTTPStatus` class provides constants for most of the HTTP status codes.
Status code 421 was added by the RFC 7540 (HTTP/2), and is not part of this class. It is described as follows:

> The 421 (Misdirected Request) status code indicates that the request
> was directed at a server that is not able to produce a response.
> This can be sent by a server that is not configured to produce
> responses for the combination of scheme and authority that are
> included in the request URI.

Would it be possible to create the corresponding constant in `http.HTTPStatus`?

For reference:
* Status code 421 in the RFC 7540:
    https://tools.ietf.org/html/rfc7540#section-9.1.2
* Documentation on the HTTPStatus class in python 3.6:
    https://docs.python.org/3/library/http.html#http-status-codes
* Another opened issue relative to the status code 451 currently pending review:
    https://bugs.python.org/issue26589

----------
components: Library (Lib)
messages: 295013
nosy: julienc
priority: normal
severity: normal
status: open
title: Add HTTP Response code 421
type: enhancement
versions: Python 3.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30553>
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