frompyobject: fix from_py_with ignored for transparent structs

[davidhewitt]

Fixes #2396

Technically this is breaking, because previously the following enum would ignore the from_py_with annotation in the first variant. Now it will be used, so rather than only accepting positive integers the first variant will now accept anything with a __len__. So there are silent changes of behaviour.

#[derive(Debug, FromPyObject, PartialEq)]
pub enum ZapEnum {
    Zip(#[pyo3(from_py_with = "PyAny::len")] usize),
    Zap(String, #[pyo3(from_py_with = "PyAny::len")] usize),
}

However, on balance I would hope that this fixes any bugs users have rather than breaks stuff! I would defend fixing this is the correct choice.