docs: update Using Python from Rust code to compile

[Mossman1215]

updates the "Using Python from Rust" example in the readme to compile with the current library version

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[davidhewitt]

Thanks for the PR! So part of the problem here is that we test the in-development PyO3 version against the README, and we have just gone through a big renaming which has created the pain here. The README currently does compile against PyO3 main, but we don't signal this very well.

I'm open to ideas about how we make this process better. It's important we check the README compiles, but checking it against the lastest release rather than main is... harder 😂

[Mossman1215]

okay I find that as a newcomer unusual so some sort of note on the readme that the quickstart guide assumes you're running the dev branch is fine by me.