Add missing magic properties to TypedDict (#6203)

stdlib/typing_extensions.pyi

@@ -52,6 +52,9 @@ def IntVar(name: str) -> Any: ...  # returns a new TypeVar
 
 # Internal mypy fallback type for all typed dicts (does not exist at runtime)
 class _TypedDict(Mapping[str, object], metaclass=abc.ABCMeta):
+    __required_keys__: frozenset[str]
+    __optional_keys__: frozenset[str]
+    __total__: bool
     def copy(self: _T) -> _T: ...
     # Using NoReturn so that only calls using mypy plugin hook that specialize the signature
     # can go through.