[3.13] gh-118934: Fix PyEval_GetLocals docs (PEP 667) (GH-119934)

PEP 667's description of the planned changes to PyEval_GetLocals
was internally inconsistent when accepted, so the docs added for
gh-74929 didn't match either the current behaviour or the intended
behaviour once gh-118934 is fixed.

This PR updates the documentation and 3.13 What's New to match the
intended behaviour (once gh-118934 is fixed).

It also tidies up lingering references to `f_locals` always being a
dictionary (this hasn't been true since at least when custom
namespace support for class statement execution was added)
(cherry picked from commit fd6cd621e0cce6ba2e737103d2a62b5ade90f41f)

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>