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 <[e-mail]>
 - Valentin Gatien-Baron <[e-mail]>
 - Yuya Nishihara <[e-mail]>

rust/hg-cpython/src/ref_sharing.rs

@@ -1,9 +1,24 b''
-// macros.rs
+// ref_sharing.rs
 //
 // Copyright 2019 Raphaël Gomès <[e-mail]>
 //
-// This software may be used and distributed according to the terms of the
-// GNU General Public License version 2 or any later version.
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to
+// deal in the Software without restriction, including without limitation the
+// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+// sell copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+// IN THE SOFTWARE.
 
 //! Macros for use in the `hg-cpython` bridge library.