Change type.__dict__ to a proper attribute (#14951)

Author: srittau

stdlib/@tests/stubtest_allowlists/common.txt

@@ -237,7 +237,6 @@ asyncio.locks.Condition.release
 builtins.memoryview.__contains__  # C type that implements __getitem__
 builtins.reveal_locals  # Builtins that type checkers pretends exist
 builtins.reveal_type  # Builtins that type checkers pretends exist
-builtins.type.__dict__  # read-only but not actually a property; stubtest thinks it's a mutable attribute.
 
 # The following CodecInfo properties are added in __new__
 codecs.CodecInfo.decode

stdlib/@tests/test_cases/builtins/check_type.py

@@ -0,0 +1,4 @@
+class Meta(type): ...
+
+
+call = Meta.__dict__["__call__"]

stdlib/builtins.pyi

@@ -42,6 +42,7 @@ from typing import (  # noqa: Y022,UP035
     Any,
     BinaryIO,
     ClassVar,
+    Final,
     Generic,
     Mapping,
     MutableMapping,
@@ -189,8 +190,9 @@ 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__: Final[types.MappingProxyType[str, Any]]  # type: ignore[assignment]
     @property
     def __dictoffset__(self) -> int: ...
     @property