Remove d2to1

See merge request !2

d2to1 is deprecated (see https://github.com/embray/d2to1) and no longer
works with setuptools >= 68.0.0, so this project's wheel can no longer
be built:

  Traceback (most recent call last):
    [...]
    File "/tmp/pip-build-env-mt7emmq3/overlay/lib/python3.12/site-packages/setuptools/dist.py", line 674, in _finalize_setup_keywords
      ep.load()(self, ep.name, value)
    File "/tmp/pip-build-env-mt7emmq3/normal/lib/python3.12/site-packages/d2to1/core.py", line 30, in d2to1
      from setuptools.dist import _get_unpatched
  ImportError: cannot import name '_get_unpatched' from 'setuptools.dist' (/tmp/pip-build-env-mt7emmq3/overlay/lib/python3.12/site-packages/setuptools/dist.py). Did you mean: 'get_unpatched'?

Port to plain modern setuptools.

Configure Renovate

See merge request !1

If True, results of the signal handler task will not be written to any
result store to not produce large amounts of irrelevant results.

Errors will still be stored.

Fix travis matrix

Fingers Crossed!

djcelery is only required for using the django result backend, or
admin, neither of which we are using during tests