Handle more platforms in simple if statements

[jakebailey]

Followup to #569. We now check if sys.platform is win32 or not, which handles almost case in typeshed, but there are more platforms we could be checking. See: https://docs.python.org/3/library/sys.html#sys.platform

Namely, darwin: https://github.com/python/typeshed/blob/de1755b263ed0b80aba35f043550c91a508c3d08/stdlib/3/stat.pyi#L68