Update README.md adding setuptools installation for Mac

[patrickreinan]

What does this PR do?

Include a pip install setuptools instruction for MacOS environments

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [x] Did you read the contributor guideline, Pull Request section?
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes?
• [ ] Did you write any new necessary tests?

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