Add macOS as CI target.

[sodul]

We have a test failure on Windows so this will have to be addressed separately.

[sodul]

On Windows:

    def __new__(cls, *args, **kwargs):
        if cls is Path:
            cls = WindowsPath if os.name == 'nt' else PosixPath
        self = cls._from_parts(args, init=False)
        if not self._flavour.is_supported:
>           raise NotImplementedError("cannot instantiate %r on your system"
                                      % (cls.__name__,))
E           NotImplementedError: cannot instantiate 'PosixPath' on your system

c:\hostedtoolcache\windows\python\3.8.10\x64\lib\pathlib.py:1044: NotImplementedError
=========================== short test summary info ===========================
FAILED tests/test_main.py::test_defaults - NotImplementedError: cannot instan...

https://github.com/clumio-oss/gray/actions/runs/7575939826/job/20633665303

This is probably easily fixable but I do not have access to a Windows machine to look into it more deeply.

[sodul]

Note that I fixed the whitespace so look at the diff with whitespace hidden.