bpo-37284: Add note to sys.implementation doc (GH-14328)


Add a brief note to indicate that any new required attributes must go through the PEP process.

https://bugs.python.org/issue37284
(cherry picked from commit 52693c10e82622d883433b779a45d0bd792f17ed)

Co-authored-by: Giovanni Cappellotto <gcappellotto@fb.com>
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index a33e796..09a987c 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -883,6 +883,10 @@
 
    .. versionadded:: 3.3
 
+   .. note::
+
+      The addition of new required attributes must go through the normal PEP
+      process. See :pep:`421` for more information.
 
 .. data:: int_info