Convert unit tests to Pytest

[NoodlesNZ]

This PR modernizes the testing framework replacing the unittest framework with pytest. pytest provides clearer assertions, powerful fixtures, and an extensive plugin ecosystem, which will make tests easier to write, maintain, and extend going forward.

[Copilot]

Pull Request Overview

Modernize tests by migrating from unittest to pytest for clearer assertions and better fixtures, and update packaging to poetry-core.

• Replace unittest.TestCase classes and assertions with plain pytest-style tests and assertions
• Update exception assertions to pytest.raises and remove unittest-only helpers
• Update pyproject.toml to use poetry-core and add pytest dependency

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
tests/test_pytmfunc.py	Convert unittest assertions and subTest blocks to pytest style; minor refactors in test bodies.
tests/test_private_func.py	Convert unittest assertions to pytest style; adjust exception assertions; minor cleanup.
pyproject.toml	Switch build backend to poetry-core; reorder Python constraints; add pytest to dependencies.

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