fix: Add overload for task without args (#54)

This add overload for following case:

```
@app.task
def task_func():
     ...
```

Refs #51

celery-stubs/app/base.pyi

@@ -107,6 +107,8 @@ class Celery:
     def start(self, argv: Optional[List[str]] = ...) -> NoReturn: ...
     def worker_main(self, argv: Optional[List[str]] = ...) -> NoReturn: ...
     @overload
+    def task(self, fun: Callable[..., Any]) -> CeleryTask: ...
+    @overload
     def task(
         self,
         *,