[red-knot] Typeshed sync and `sys.platform` fixes (#15492)
## Summary
The next sync of typeshed would have failed without manual changes
anyway, so I'm doing one manual sync + the required changes in our
`sys.platform` tests (which are necessary because of my tiny typeshed PR
here: https://github.com/python/typeshed/pull/13378).
closes #15485 (the next run of the pipeline in two weeks should be fine
as the bug has been fixed upstream)
