try to debug codspeed issue

[davidhewitt]

...

[davidhewitt]

Ok so I think I see what was going on, it looks like probably what was happening is that pip install was installing into the virtualenv and hitting PyO3/maturin#490

I'm not sure why this wasn't a problem initially. I wonder if it was a cache issue related to the same problems we were having recently with rustup conflicts.

I removed the pyo3_pytests/__init__.py to use a pure-rust maturin project, which should fix the problem. I also took the opportunity to refactor as per #3536

[davidhewitt]

🎉 looks like that's fixed it. I'll merge this as I think there's nothing much to review here and it improves CI for the rest of the reviews.