gh-118934: Fix PyEval_GetLocals docs (PEP 667) (#119932)

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)

Doc/reference/datamodel.rst

@@ -1347,13 +1347,13 @@ Special read-only attributes
        ``object.__getattr__`` with arguments ``obj`` and ``"f_code"``.
 
    * - .. attribute:: frame.f_locals
-     - The dictionary used by the frame to look up
+     - The mapping used by the frame to look up
        :ref:`local variables <naming>`.
        If the frame refers to an :term:`optimized scope`,
        this may return a write-through proxy object.
 
        .. versionchanged:: 3.13
-          Return a proxy for functions and comprehensions.
+          Return a proxy for optimized scopes.
 
    * - .. attribute:: frame.f_globals
      - The dictionary used by the frame to look up