rust-cpython: change license of ref_sharing.rs to MIT
Since we plan to upstream this feature, it's better to continue further
refactoring under the same license as rust-cpython crate.
According to the file history, copyright holders are:
- Raphaël Gomès <rgomes@octobus.net>
- Valentin Gatien-Baron <valentin.gatienbaron@gmail.com>
- Yuya Nishihara <yuya@tcha.org>