Allow single element tuple as return type (small fix)

[LeoWherle]

Example of previously non-working case:

use pyo3_stub_gen::derive::{gen_stub_pyclass, gen_stub_pymethods};
use pyo3::prelude::*;

#[gen_stub_pyclass]
struct MyStruct(u32);

#[gen_stub_pymethods]
impl MyStruct {
    fn __getnewargs__(&self) -> PyResult<(u32,)> {
        Ok((0,))
    }
}

[termoshtt]

Thanks PR! But this has been fixed as a part of #208