Revert GitHub Actions for Python 3.8 to macos-latest

[abravalheri]

Summary of changes

The issue might have been solved in actions/setup-python#850 (comment)

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[abravalheri]

Or maybe not...

This error message is suspicious:

subprocess.CalledProcessError: Command '['/Library/Frameworks/Python.framework/Versions/3.8/bin/python', '-c', '\nimport runpy\nimport sys\nsys.path = [\'/var/folders/3m/p59k4qdj0f17st0gn2cmj3640000gn/T/tmpezbv7_yj/setuptools-56.0.0-py3-none-any.whl\', \'/var/folders/3m/p59k4qdj0f17st0gn2cmj3640000gn/T/tmpezbv7_yj/pip-21.1.1-py3-none-any.whl\'] + sys.path\nsys.argv[1:] = [\'install\', \'--no-cache-dir\', \'--no-index\', \'--find-links\', \'/var/folders/3m/p59k4qdj0f17st0gn2cmj3640000gn/T/tmpezbv7_yj\', \'setuptools\', \'pip\']\nrunpy.run_module("pip", run_name="__main__", alter_sys=True)\n']' returned non-zero exit status 1.

Specially the setuptools-56.0.0-py3-none-any.whl part.