Remove webcolors as a dependency

[rich-iannone]

The webcolors package is not needed but is currently listed as a required dependency in pyproject.toml. I believe it was added during a PR but in the resolution of that work we didn't end up needing webcolors.

See https://github.com/search?q=repo%3Aposit-dev%2Fgreat-tables%20webcolors&type=code where there is only a single reference to webcolors in the codebase.