Fx self-check on macOS

[ilevkivskyi]

See #22124 (comment)

[cdce8p]

Hmm, I'm still getting the warning on macos.

Would this still resolve the original issue?

    if (
        PSUTIL_AVAILABLE
        and (psutil.LINUX or psutil.WINDOWS or psutil.FREEBSD)
        and sys.platform != "darwin"
    ):

[ilevkivskyi]

Oh, this is annoying, my trick only works at top-level. I guess we would need to use the if...

[cdce8p]

The self check for macos is working now. Thanks @ilevkivskyi

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
+ tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports