[3.12] gh-115528: Update language reference for PEP 646 (GH-121181) (#124633)

[3.12] gh-115528: Update language reference for PEP 646 (GH-121181) (#124633)

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

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 7d3497f617edf77cb6ead6f5e62bce98d77b9ab8)

Co-authored-by: Matthew Rahtz <matthew.rahtz@gmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>