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, we use `CString::new()` that returns a `result<CString, NulError>`. `CString` is a struct that wraps around a `Vec<u8>`. We then convert this using `as_c_str()` which returns a `&CStr`, which now satisfies `PyModule::from_code_bound()` parameter rules.
The other changes to call, new, import and empty are only renamed to keep the syntax up to date.