gh-102780: Fix uncancel() call in asyncio timeouts (#102815)

Also use `raise TimeOut from <CancelledError instance>` so that the CancelledError is set
in the `__cause__` field rather than in the `__context__` field.

Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>