Running in virtualenvs on Windows

[davidhewitt]

See also #1835

If I create a new virtualenv on Windows, install tomli into it, and run the following program:

use pyo3::prelude::*;

fn main() -> PyResult<()>{
    Python::with_gil(|py| {
        let m = PyModule::import(py, "tomli")?;
        Ok(())
    })
}

Then this fails to import tomli, despite building correctly with the virtualenv. On linux (at least Ubuntu 20.04) the equivalent works correctly.

I suspect we could investigate how pyo3::prepare_freethreaded_python sets up the interpreter - presumably on Windows it fails to detect the virtualenv, but on Linux it's fine.