Test install_dependencies.sh script on macOS

[pamolloy]

Adding job to the Github Workflow as follows led to various issues with Homebrew:

  install-macos:
    runs-on: macos-latest
    steps:
      - uses: actions/checkout@v2
      - run: source/_static/install_dependencies.sh

Attempting to install python3 and git using Homebrew caused the following errors:

Error: python@3.9 3.9.1 is already installed
...
Error: git 2.29.2 is already installed

Attempting to upgrade those packages caused the following errors:

==> Installing python@3.9
==> Pouring python@3.9-3.9.1_6.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

It seems like Python and git are already installed by some other means than homebrew, perhaps using Xcode?

[pamolloy]

I'm hopefully going to pick up a Intel Mac mini this afternoon, which should help debugging this.