PR: Drop unittest usage in favour of pytest.

[KelSolaar]

Summary

This PR is a first initial pass at dropping unittest in favour of pytest with the intent of using parameterised tests with #1244.

I used a mix of pytestify and unittest2pytest to do most of the conversion and finished manually.

Preflight

Code Style and Quality

• Unit tests have been implemented and passed.
• Pyright static checking has been run and passed.
• Pre-commit hooks have been run and passed.
• [N/A] New transformations have been added to the Automatic Colour Conversion Graph.
• [N/A] New transformations have been exported to the relevant namespaces, e.g. colour, colour.models.

Documentation

• [N/A] New features are documented along with examples if relevant.
• [N/A] The documentation is Sphinx and numpydoc compliant.

[coveralls]

coverage: 99.815% (-0.01%) from 99.829%
when pulling eff1ef2 on feature/pytest
into 7ecfb43 on develop.