Conflict between pyproject.toml and pipenv install

pipenv install
Pipfile.lock (19a47c) out of date, updating to (718822)…
Locking [dev-packages] dependencies…
✔ Success! 
Locking [packages] dependencies…
✔ Success! 
Updated Pipfile.lock (19a47c)!
Installing dependencies from Pipfile.lock (19a47c)…
An error occurred while installing -e .! Will try again.
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Installing initially failed dependencies…
[pipenv.exceptions.InstallError]: ['Obtaining file:///home/moul/projects/silkaj']
[pipenv.exceptions.InstallError]: ['ERROR: Error installing \'file:///home/moul/projects/silkaj\': editable mode is not supported for pyproject.toml-style projects. pip is processing this project as pyproject.toml-style because it has a pyproject.toml file. Since the project has a setup.py and the pyproject.toml has no "build-backend" key for the "build_system" value, you may pass --no-use-pep517 to opt out of pyproject.toml-style processing. See PEP 517 for details on pyproject.toml-style projects.']
ERROR: ERROR: Package installation failed...

Introduced in !76 (merged).