Change type.__dict__ to a proper attribute

Closes: #11033

Author: srittau

stdlib/builtins.pyi

@@ -189,8 +189,9 @@ class type:
     __bases__: tuple[type, ...]
     @property
     def __basicsize__(self) -> int: ...
-    @property
-    def __dict__(self) -> types.MappingProxyType[str, Any]: ...  # type: ignore[override]
+    # type.__dict__ is read-only at runtime, but that can't be expressed currently.
+    # See https://github.com/python/typeshed/issues/11033 for a discussion.
+    __dict__: ClassVar[types.MappingProxyType[str, Any]]  # type: ignore[override]
     @property
     def __dictoffset__(self) -> int: ...
     @property