Add type stub for colour package

[absolutely-vivid]

This PR adds type stubs for the colour package.

There were a few small issues when writing the stubs that might need changes before merging:

• stubtest didn't like the type I wrote for colour.Color.set_hex_l, it said the runtime type was Callable[[Any], Any]. I think that's because set_hex_l isn't defined as a method, but as a class variable with a lambda. I added colour.COlor.set_hex_l to the allowlist
• colour.Color.__eq__'s implementation can return (not raise) NotImplemented. Mypy and pyright both disallow that, so I set a noqa
• Mypy complains about the colour.Color.hash_or_str overload, saying overload 1 is wider than overload 2. However Hashable is narrower than object (set is object but not hashable), so that shouldn't be the case. I ignored with a noqa comment

If there are more idiomatic ways to fix any of those, let me know and I can make the changes.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉