The main setuptools PEP 517 backend is intended for
explicit usage in `pyproject.toml`, when the project
authors can ensure that their `setup.py` runs without
that directory being implicitly on `sys.path`.
For implicit usage, setuptools now offers a separate
legacy backend that more closely mimics direct
execution of the `setup.py` script.
The message looks like this:
File "setup.py" not found. Directory cannot be installed in editable
mode: <absolute-dir-path>
(A "pyproject.toml" file was found, but editable mode currently
requires a setup.py based build.)