Recognize Iterator as Hashable

[aryansk]

Fixes #21813

Summary

• Add hash to typing.Iterator in the stdlib typing stub.
• Mirror the protocol member in the unit-test typing fixture.
• Add a regression check showing Iterator[str] satisfies Hashable while Iterable[str] does not.

Tests

• runtests.py check-protocols.test: 200 passed, 1 skipped.
• The issue reproducer now reports no errors.

This PR was prepared with AI assistance, as permitted by the contributor guidance, and the implementation and test results were reviewed locally.

[aryansk]

Thanks for the primer report. The original stub change was too broad: it made Iterator.__hash__ mandatory and caused downstream regressions. I amended the PR at a0f382aa2763f4ec94bb3968dd3a2638614b8661 to handle the standard Iterator-to-Hashable relation in subtype checking, with regression coverage for the issue reproducer and an unhashable Iterator subclass. Local check-protocols.test passes (200 passed, 1 skipped); hosted checks are rerunning.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas-stubs (https://github.com/pandas-dev/pandas-stubs)
+ tests/frame/test_indexing.py:458: error: Unused "type: ignore[index]" comment  [unused-ignore]