Running in virtualenvs on Windows

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.

Start by reproducing the Windows virtualenv case from the issue, comparing it with the stated Ubuntu behavior. Read the interpreter setup around pyo3::prepare_freethreaded_python and investigate how the virtualenv is detected. Done means a Rust program can import tomli from a Windows virtualenv as described.