Remove unused typing.cast imports

[ThomasWaldmann]

The Test workflow currently fails on main at the Lint (ruff) step:

F401 [*] `typing.cast` imported but unused
  --> examples/hello.py:30:20
  --> examples/hello_asyncio.py:31:20
  --> test/test_fs.py:13:20
Found 3 errors.

The imports became unused in b241c54 ("Turn FileHandleT and InodeT from
NewTypes into type aliases"), which removed the cast() calls but left the
imports behind. Since fail-fast: true, this cancels the whole matrix, so no
tests have actually run on main since then.

With the three lines removed, the full matrix is green again (ruff, ruff
format, mypy, pyright, tests, docs on 3.10-3.14).

🤖 Generated with Claude Code