bpo-29613: Added support for SameSite cookies (GH-6413)

* bpo-29613: Added support for SameSite cookies

Implemented as per draft
https://tools.ietf.org/html/draft-west-first-party-cookies-07

* Documented SameSite

And suggestions by members.

* Missing space :(

* Updated News and contributors

* Added version changed details.

* Fix in documentation

* fix in documentation

* Clubbed test cases for same attribute into single.

* Updates

* Style nits + expand tests

* review feedback
diff --git a/Lib/http/cookies.py b/Lib/http/cookies.py
index 7e0259e..4a44db8 100644
--- a/Lib/http/cookies.py
+++ b/Lib/http/cookies.py
@@ -281,6 +281,7 @@
         "secure"   : "Secure",
         "httponly" : "HttpOnly",
         "version"  : "Version",
+        "samesite" : "SameSite",
     }
 
     _flags = {'secure', 'httponly'}