`Python::allow_threads` is unsound in the presence of `send_wrapper`.

It allows smuggling Python types into the closure:

use pyo3::prelude::*;
use pyo3::types::PyString;
use send_wrapper::SendWrapper;

fn main() {
    Python::with_gil(|py| {
        let string = PyString::new(py, "foo");

        let wrapped = SendWrapper::new(string);

        py.allow_threads(|| {
            let smuggled: &PyString = *wrapped;
            println!("{:?}", smuggled);
        });
    });
}

Results in
error: process didn't exit successfully: target\debug\my_module.exe (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION)

So I'm not sure whose fault it is:

• Ours, for using Send for something it wasn't meant to.
• send_wrapper's, for assuming others didn't put arbitrary restrictions on Send.