drop d2to1

[branchv]

broken on modern setuptools:

$ pip3 install --no-binary=:all: shyaml
...
× Preparing metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [33 lines of output]
      Traceback (most recent call last):
      ...
        File "/private/var/folders/1r/p5nhsx151fd7j63p9r1zz4kw0000gn/T/pip-build-env-xrqb__gx/normal/lib/python3.11/site-packages/d2to1/core.py", line 30, in d2to1
          from setuptools.dist import _get_unpatched
      ImportError: cannot import name '_get_unpatched' from 'setuptools.dist' (/private/var/folders/1r/p5nhsx151fd7j63p9r1zz4kw0000gn/T/pip-build-env-xrqb__gx/overlay/lib/python3.11/site-packages/setuptools/dist.py)
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

[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 shyaml 0.6.2 sdist fails to build against modern setuptools (d2to1/get_unpatched crash), so pip install --no-binary :all: shyaml is broken today. Found while build-checking the top-5000 PyPI packages for setuptools-removal breakage — +1 for merge and release.