I think the failure is coming from this line of the mypy tests: https://github.com/python/mypy/blob/5cfb0561e624b79365ac5333cbde7ff500249740/test-data/stdlib-samples/3.2/test/support.py#L476

The test isn't specifically for platform, so the usage of a platform-specific function is invalid. Does anyone with more knowledge of mypy test suite know?

Ugh, that's annoying. I think I might have actually tried to make this change before.

mypy seems pretty receptive to removing stdlib samples that are showing their age, see python/mypy#8838 (comment). I think test.support might be tricky to remove though, in which case maybe just change the check to be against sys.platform (or type-ignore that line or unconditionally define those constants).

Yes, probably easiest to make the mypy code use sys.platform instead as @hauntsaninja said. I can merge a mypy PR doing that.

Opened a PR to mypy as python/mypy#8916