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?

Start with the referenced declarations in stdlib/builtins.pyi and stdlib/types.pyi, then read PEP 667's description of FrameType.f_locals and compare it with the runtime examples in the issue. Done means determining and documenting compatible types for both locals() and f_locals, including whether a structural mapping protocol is needed.