macOS backend should not be viable when API is unavailable

[jaraco]

In #525, I note that when the API is broken, the macOS backend still reports as viable. Instead, it should be non-viable when the API is invalid.

[jaraco]

Oh, this is interesting, and will make it difficult to detect the current macOS version:

$ py -3.8 -c "import platform; print(platform.mac_ver()[0])"
12.0.1
$ py -3.7 -c "import platform; print(platform.mac_ver()[0])"
10.16