pythongh-136992: Add "None" as valid SameSite value as per RFC 6265bis (python#137040)

The "SameSite" attribute defined in RFC 6265bis [1] allows the "Strict", "Lax" and "None"
enforcement modes. We already documented "Strict" and "Lax" as being valid values
but "None" was missing from the list. While the RFC has not been formally approved,
modern browsers support the "None" value [2, 3] thereby making sense to document it.

[1]: https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-rfc6265bis
[2]: https://developers.google.com/search/blog/2020/01/get-ready-for-new-samesitenone-secure
[3]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Set-Cookie#none

---------

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: 2 people

Doc/library/http.cookies.rst

@@ -148,9 +148,12 @@ Morsel Objects
    in HTTP requests, and is not accessible through JavaScript. This is intended
    to mitigate some forms of cross-site scripting.
 
-   The attribute :attr:`samesite` specifies that the browser is not allowed to
-   send the cookie along with cross-site requests. This helps to mitigate CSRF
-   attacks. Valid values for this attribute are "Strict" and "Lax".
+   The attribute :attr:`samesite` controls when the browser sends the cookie with
+   cross-site requests. This helps to mitigate CSRF attacks. Valid values are
+   "Strict" (only sent with same-site requests), "Lax" (sent with same-site
+   requests and top-level navigations), and "None" (sent with same-site and
+   cross-site requests). When using "None", the "secure" attribute must also
+   be set, as required by modern browsers.
 
    The attribute :attr:`partitioned` indicates to user agents that these
    cross-site cookies *should* only be available in the same top-level context