Remove unmaintained d2to1 during setup.

[tjni]

I am updating setuptools in nixpkgs to v68 and found that d2to1 does not work anymore.

Closes #60.

[sethc555]

Independent verification: with this PR checked out, python -m build (sdist → wheel) succeeds in a clean python:3.12 container with current setuptools. Without it, the published colour 0.1.5 sdist fails to build against modern setuptools (the d2to1 path crashes in get_unpatched), so pip install --no-binary :all: colour is broken today. Found while build-checking the top-5000 PyPI packages for setuptools-removal breakage — +1 for merge and release. (Note: verification needs a git checkout with tags for the autogen version machinery.)