The types of `builtins.locals` and `types.FrameType.f_locals` aren't correct

builtins.locals and types.FrameType.f_locals are currently typed with dict[str, Any], which is incorrect:

• https://github.com/python/typeshed/blob/85a787bba3057f1df44188b4c815069d2d931557/stdlib/builtins.pyi#L1547

  Counterexample:

  from collections import UserDict
  
  class A(type):
      @classmethod
      def __prepare__(cls, name, bases): 
          return UserDict()
  
      def __new__(mcs, name, bases, namespace, /, **kw): 
          return super().__new__(mcs, name, bases, dict(namespace), **kw)
  
  class B(metaclass=A):
      assert isinstance(locals(), dict)  # AssertionError
  

• https://github.com/python/typeshed/blob/85a787bba3057f1df44188b4c815069d2d931557/stdlib/types.pyi#L585-L586

  Counterexample as a consequence of PEP 667 (Python >= 3.13):

  from inspect import currentframe
  
  def f():
      print(type(currentframe().f_locals))  # <class 'FrameLocalsProxy'>  (runtime type is `_collections_abc.framelocalsproxy`; not in `.pyi` stubs yet)
      assert isinstance(currentframe().f_locals, dict)  # AssertionError
  
  f()
  

However, I'm not sure what the actual type should be. From what I can gather, the type should implement a mapping interface, with a few (but not all?) MutableMapping methods (PEP 667 describes what these mutable mapping methods are for FrameType.f_locals, but I'm not sure what's exactly expected of builtins.locals()).

In addition, collections.abc.Mapping and collections.abc.MutableMapping themselves are unsuitable, because they're nominal types, although this could be fixed by defining a Protocol which looks like a Mapping.

So, what should the types of builtins.locals and types.FrameType.f_locals be?

stdlib/builtins.pyi と stdlib/types.pyi にある参照されている宣言から始め、次に PEP 667 の FrameType.f_locals の説明を読み、issue のランタイム例と比較します。locals() と f_locals の両方について互換性のある型を決定して文書化し、構造的マッピングプロトコルが必要かどうかも判断できれば完了です。