Add mypyc_attr to the mypy_extensions lib-stub

test-data/unit/lib-stub/mypy_extensions.pyi

diff --git a/test-data/unit/lib-stub/mypy_extensions.pyi b/test-data/unit/lib-stub/mypy_extensions.pyi
index 8fb5942..a0e01c3 100644
--- a/test-data/unit/lib-stub/mypy_extensions.pyi
+++ b/test-data/unit/lib-stub/mypy_extensions.pyi
@@ -42,4 +42,6 @@
 # when a Type[_T] is expected, so we can't give it the type we want.
 def trait(cls: Any) -> Any: ...
 
+def mypyc_attr(*attrs: str, **kwattrs: object) -> Callable[[_T], _T]: ...
+
 class FlexibleAlias(Generic[_T, _U]): ...