PR: Make *OpenImageIO* required and *ImageIO* optional.

[KelSolaar]

Summary

This PR makes OpenImageIO required and Imageio optional thanks to the new Python wheels.

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

• New features are documented along with examples if relevant.
• The documentation is Sphinx and numpydoc compliant.

[coveralls]

coverage: 99.346% (+0.01%) from 99.334%
when pulling 09a0593 on feature/openimageio
into 42e9e0e on develop.