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>