[PNHD]

Fixes #31.

Include the Python test files in source distributions so downstream packagers can run the existing suite from the published source archive. This adds one MANIFEST.in rule; runtime code and tests are unchanged.

Validation on Windows with Python 3.11.9 and setuptools 65.5.0:

• A fresh source distribution from the base omits both tracked test files, so the package-content assertion fails.
• With the manifest rule, both test files are included with their original bytes and the same assertion passes.
• All 55 existing unittest cases pass from the unpacked candidate source distribution.
• Comparing the source archives finds only the two test files added, with no removals or other common-file changes apart from MANIFEST.in and generated SOURCES.txt. Package metadata, license, README and py.typed are preserved.
• git diff --check passes.

The configured flake8 and mypy checks retain the same five F401 findings and three missing-six-stub diagnostics on base and head. No checker settings were changed.

Other Python versions, PyPy and hosted CI were not run.