Add bitspack tests and pytest dev dependency

[Copilot]

Restores tests/bitspack_test.py (removed in b47b47f) with code style updated to match the existing test conventions, and adds pytest to the dev optional dependencies.

Changes

• tests/bitspack_test.py: Re-added bitspack round-trip tests, updated to current conventions:
  • Import from qmp.utility.bitspack (was qmb.bitspack)
  • Tests grouped into classes (TestPackUnpackIntSize1/2/4/8) with per-method docstrings, matching hamiltonian_test.py style
• pyproject.toml: Added pytest>=8.4.1 to [project.optional-dependencies] dev

Original prompt

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This section details on the original issue you should resolve

<issue_title>添加测试</issue_title>
<issue_description>将 b47b47f^ 的 tests/bitspack_test.py 添加回来, 但需要更新代码风格, 保持和现有的一致. 另外, 把pytest加dev optional dep里
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 添加测试 #209

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