Python test code refactoring - part 2

[solidpixel]

This PR refactors the Python test code to make it PEP8 compliant, add mypy typing, and fix most Pylint issues. Other style changes make it consistent with Python libraries used in other graphics-related Arm open-source projects.

TODO:

• Refactor all the Python code in ./Test and subdirectories.
• Add more tests to Actions to actually run the Python scripts with various flags to check they work (or at least don't crash).
• Update Testing.md - it's out of date.
• Drop support for < 5.0 compressors in the test suite, including dropping reference data as it's large.