Fix the build error of GitHub Actions for macOS.
Fix the build error of GitHub Actions for macOS.

* https://github.com/google/mozc/actions/runs/2286074173
* readPlist and writePlistToBytes have been removed since Python 3.9
  + https://docs.python.org/3.9/whatsnew/3.9.html#removed

PiperOrigin-RevId: 447302870

google/mozc,google/mozc,fcitx/mozc,fcitx/mozc,google/mozc,fcitx/mozc,google/mozc,google/mozc,fcitx/mozc,fcitx/mozc