Revert soft-duplicated for Py::new

[kngwyu]

Since PyCell is registered to GILPool, Py<T>::new can be more efficient especially when the object is directly returned to Python.
E.g.,

#[pyfunction] 
fn new_class(py: Python) -> Py<MyClass> {
    Py::new(py, ...)
}

[davidhewitt]

That's a fair point 👍 . Perhaps we should update docs to recommend Py::new instead of PyCell::new in most cases?

[kngwyu]

Perhaps we should update docs to recommend Py::new instead of PyCell::new in most cases?

Yeah, but I'm not sure where they are