Updated the pyo3 module in chronik-plugin-impl/tests/Cargo.toml.
The old syntax `PyModule::from_code_bound()` takes in a `&str`. With the new change to `PyModule::from_code()`, we need to take in a `&CStr`.
Therefore, created a function that converts from `&str` -> `&CStr`.
The other changes to call, new, import and empty are only renamed to keep the syntax up to date.