python: make PEP 585/604 annotations parse on 3.8 and 3.9

[congwang-mk]

Summary

pyproject.toml declares requires-python = ">=3.8", but the source and tests use PEP 604 (str | None) and PEP 585 (dict[str, int]) annotations at module load time, which only parse on 3.10+ and 3.9+ respectively. Any import sandlock on 3.8/3.9 crashed at collection. The 3.10/3.11/3.12 CI matrix hid it.

Add from __future__ import annotations to every file that uses the modern syntax and isn't already shimmed (16 files: one src module, the rest tests). PEP 563 postpones annotation evaluation to runtime lookups, so the literal str | None and dict[str, int] parse fine on every supported version. Verified no runtime usage of these forms (no isinstance(x, str | None), no typing.cast, no typing.get_type_hints) so the future import alone is sufficient.

Test plan

• Python 3.8: python3 -m pytest tests/ (319 / 321 pass; the 2 failures are pre-existing: optional mcp extra missing, and a sub-sandbox PYTHONPATH quirk, both unrelated to the syntax change)
• Python 3.9: same — 296 / 297 pass with the same single pre-existing failure.
• Module imports cleanly on both versions: python3.8 -c "import sandlock" and python3.9 -c "import sandlock" succeed.

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