Add utf-8 encoding to opens in release.py (#4752)

Git by default uses utf-8 file encodings, so these files will always be utf-8. On a windows system without PYTHONUTF8 set, these opens will fail (normal black doesn't have this issue because it detects file encodings automatically).

This PR makes it so that the opens always use utf-8 mode, so the script will always work on windows systems.