- The `tests/__init__.py` create/delete thing seems to have been a
  workaround for something that's no longer relevant, so just remove it.
- The `test-all` target needed to depend on `clean` so that `build/lib`
  wouldn't stick around and confuse pytest, and the recipe needed to be
  pruned down to just `tox` because otherwise we'd run the quality checks
  twice. (`quality` is in the tox.ini default env list.)