arrow

ARROW-10993: [CI][macOS] Fix Python 3.9 installation by Homebrew

https://github.com/apache/arrow/runs/1579780011#step:4:520

    ==> Pouring python@3.9-3.9.1_1.catalina.bottle.tar.gz
    Error: The `brew link` step did not complete successfully
    The formula built, but is not symlinked into /usr/local
    Could not symlink bin/2to3
    Target /usr/local/bin/2to3
    already exists. You may want to remove it:
      rm '/usr/local/bin/2to3'

    To force the link and overwrite all conflicting files:
      brew link --overwrite python@3.9

    To list all files that would be deleted:
      brew link --overwrite --dry-run python@3.9

    Possible conflicting files are:
    /usr/local/bin/2to3 -> /Library/Frameworks/Python.framework/Versions/2.7/bin/2to3

Closes #8976 from kou/ci-homebrew

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>