PR: Use colour.algebra.spow in *ITU-R BT.1886* colour component transfer functions

[KelSolaar]

Summary

This PR ensures that colour.algebra.spow is used in ITU-R BT.1886 colour component transfer functions to avoid NaN generation when negative values are passed.

References #1295

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: 99.835%. remained the same
when pulling ed810be on feature/spow_bt1886
into dc87522 on develop.