Address review feedback and restore CI compatibility

The Python package workflow failed during import on Python 3.7 and 3.8 because subprocess.Popen was subscripted in an eagerly evaluated annotation. It also reported mypy errors caused by the invariant IterableList type variable and an overly narrow IndexFileSHA1Writer stream type; lint reported an unused IO import.

Quote review feedback: IterableList variance no longer matched its subtype comment; the shutil.rmtree callback was narrower than the typeshed contract; and expand_path overloads omitted None and pathlib.Path results and did not handle Path.resolve exceptions consistently.

Restore covariance and the IO[bytes] stream type, quote the Popen annotation for old Python runtimes, correct rmtree callback types, and make expand_path return and exception behavior match the implementation. Update the cwd annotation that now correctly flows from expand_path. This addresses the review feedback and restores the lint and mypy checks.

Author: codex