Updated demo

[Michael-J-Ward]

Hi Colin!

I was looking to learn about adding coverage to a rust-pyo3 project and was delighted to come across your wonderful demo!

In following it, I had to clear out some standard bit-rot things (cargo llvm-cov --no-run is deprecated in favor of cargo llvm-cov report, for example), upgraded the dependencies (pyo3's new Bound<'_, T>) and added a few clarifications to questions that I personally had.

I have a note with these updates at the top of the README.

https://github.com/Michael-J-Ward/rust-python-coverage

Would you like me to create a PR with these updates?

If not, is it OK if I share my forked demo with people?

Hope you're doing well!

Cheers