Python 3.7 not available for macOS v14 in GitHub Actions

[kandersolar]

Until recently, macos-latest referred to version 12.7.4: https://github.com/pvlib/pvlib-python/actions/runs/8786214399/job/24108552235?pr=2006#step:1:4

Now it refers to version 14.4.1: https://github.com/pvlib/pvlib-python/actions/runs/8880987265/job/24382296170?pr=2022#step:1:4

See also this github blog post regarding macOS 14: https://github.blog/changelog/2024-04-01-macos-14-sonoma-is-generally-available-and-the-latest-macos-runner-image/

The immediate relevance for us is that python 3.7 is not available in version 14, so the macos-latest, python 3.7 matrix job is now failing: https://github.com/pvlib/pvlib-python/actions/runs/8880987265/job/24382296170

I see three alternatives:

1. Modify our testing configuration so that we stick with the older version of macOS.
2. Keep the newer version for all macOS jobs except python 3.7, and use the older version only for that.
3. Drop the python 3.7 job (see retire support for python 3.7 #1975).

[cwhanse]

My vote is for option 3, which I think is what will happen (eventually) if we choose 2 or 1.