Support Python 3.10-3.14, fix registry read_text() usage, drop unused accelerator-toolbox dependency

[gupichon]

Summary

• Fix ImportError: cannot import name 'Traversable' from 'importlib.abc' on Python 3.14 (reported by pyaml's CI): importlib.abc.Traversable was deprecated in 3.12 and removed in 3.14, so it's now imported from importlib.resources.abc with a fallback to importlib.abc for 3.10/3.11.
• Fix Registry.__getitem__, which returned str(path) instead of the Traversable/Path object, silently breaking the README's documented lattice_file.read_text() usage. It now returns the entry itself.
• Breaking change: drop the accelerator-toolbox runtime dependency, nothing in this package imports it, it's purely a file registry. Any project relying on it being pulled in transitively (e.g. pyaml) must now declare it as a direct dependency of its own.
• Bump supported range to Python 3.10-3.14 (requires-python, classifiers, ruff/mypy target versions).
• Add a pytest suite under tests/ exercising the README's documented usage (lattice/configuration access, .read_text(), descriptions) and the supported Python version range, plus a GitHub Actions workflow (unit-tests.yml) running it across Python 3.10, 3.11, 3.12, 3.13, and 3.14.

Closes #5

Test plan

• pytest passes on Python 3.10 and Python 3.14 (conda envs)
• ruff check . and ruff format --check . pass
• CI matrix (unit-tests.yml) green on the PR