fix wheels for macOS 10.13

[grlee77]

Description

closes #5465 (fixes wheels for older OS X releases)

The first commit here is from Mark's comment: #5465 (comment)

An open question that Mark raised is why we only include libomp for the Python 3.9 case on macOS? Was this intentional or an oversight?

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[grlee77]

For reference, OS X 10.13 was released in September 2017:
https://en.wikipedia.org/wiki/MacOS_version_history#Releases

[grlee77]

Is 10.13 still the oldest OS X we want to support? (The table linked in the prior comment seems to indicate that 10.14 is the oldest that is still actively maintained by Apple)

[alexdesiqueira]

Is 10.13 still the oldest OS X we want to support?

@grlee77 I'm not sure about MacOS stuff, but I don't think that keeping a version would hurt... 🙂

[alexdesiqueira]

Thank you @grlee77 and @hmaarrfk! LGTM; feel free to keep it as 10.13, or to update to 10.14 if you'd prefer.

[hmaarrfk]

Lets keep it for this PR. Thanks!

[grlee77]

Just noticed that I left "scikit-learn" in a comment, but can fix that in a followup. Trying to look into whether we need this same brew install for the other Python versions as well... (the fix here was only for the Python 3.9 job!)