[3.13] gh-115528: Update language reference for PEP 646 (GH-121181)

[miss-islington]

To recap: the objective is to make starred expressions valid in subscription,
which is used for generics: Generic[...], list[...], etc.

What is gramatically valid in such contexts? Seemingly any of the following.
(At least, none of the following throw SyntaxError in a 3.12.3 REPL.)

Generic[x]
Generic[*x]
Generic[*x, y]
Generic[y, *x]
Generic[x := 1]
Generic[x := 1, y := 2]

So introducting

flexible_expression: expression | assignment_expression | starred_item

end then switching subscription to use flexible_expression sorts that.

But then we need to field yield - for which any of the following are
apparently valid:

yield x
yield x,
yield x, y
yield *x,
yield *x, *y

Introducing a separate yield_list is the simplest way I've been figure out to
do this - separating out the special case of starred_item ,.

(cherry picked from commit 7d3497f)

Co-authored-by: Matthew Rahtz [email protected]
Co-authored-by: Jelle Zijlstra [email protected]

• Issue: PEP 646 grammar changes are not documented #115528

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