fix declarative_mixin stub (#232)

Co-authored-by: Mehdi Gmira <mgmira@wiremind.io>

Author: mehdigmira

sqlalchemy-stubs/orm/decl_api.pyi

@@ -40,7 +40,7 @@ class _stateful_declared_attr(declared_attr):
     def __init__(self, **kw: Any) -> None: ...
     def __call__(self, fn: Any): ...
 
-def declarative_mixin(cls: type) -> type: ...
+def declarative_mixin(cls: Type[_T]) -> Type[_T]: ...
 def declarative_base(
     bind: Optional[
         Any