[BUG] pyproject.toml validation breaks pdm-pep517

setuptools version

61.3.0

Python version

Python 3.10.4

OS

Gentoo Linux

Additional environment information

Tested on top of pdm-pep517 git (pdm-project/pdm-pep517@04ed8589ad4400422ac9ca4c09b3ecb0df8edfab).
pdm-pep517 bug report: https://github.com/pdm-project/pdm-pep517/issues/81

Description

pdm-pep517 happens to use setuptools internally to build projects with C extensions. However, these builds are now failing due to setuptools attempting to validate pyproject.toml according to its own rules, while it apparently uses an incompatible pdm-specific format. An example error is:

configuration error: `project` must not contain {'license-expression'} properties

Expected behavior

Not sure. Possibly skipping validation if pyproject.toml uses a different build system.

How to Reproduce

git clone https://github.com/pdm-project/pdm-pep517
cd pdm-pep517
python -m venv .venv
. .venv/bin/activate
pip install pdm
pdm run pytest tests

Output

The tests initially fail due to empty homepage:

-------------------------------------------------------- Captured stderr call ---------------------------------------------------------
/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py:102: _ExperimentalProjectMetadata: Support for project metadata in `pyproject.toml` is still experimental and may be removed (or change) in future releases.
  warnings.warn(msg, _ExperimentalProjectMetadata)
For maximum compatibility please make sure to include a `scheme` prefix in your URL (e.g. 'http://'). Given value: 
configuration error: `project.urls.homepage` must be url
Traceback (most recent call last):
  File "/tmp/pdm-pep517/tests/fixtures/projects/demo-cextension/setup.py", line 29, in <module>
    setup(**setup_kwargs)
  File "/usr/lib/python3.10/site-packages/setuptools/__init__.py", line 87, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.10/site-packages/setuptools/_distutils/core.py", line 122, in setup
    dist.parse_config_files()
  File "/usr/lib/python3.10/site-packages/setuptools/dist.py", line 854, in parse_config_files
    pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 54, in apply_configuration
    config = read_configuration(filepath, True, ignore_option_errors, dist)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 120, in read_configuration
    validate(subset, filepath)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 43, in validate
    raise error from None
ValueError: invalid pyproject.toml config: `project.urls.homepage`

After fixing it not to be empty, they fail due to pdm-specific keys, e.g.:

/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py:102: _ExperimentalProjectMetadata: Support for project metadata in `pyproject.toml` is still experimental and may be removed (or change) in future releases.
  warnings.warn(msg, _ExperimentalProjectMetadata)
configuration error: `project` must not contain {'license-expression'} properties
Traceback (most recent call last):
  File "/tmp/pdm-pep517/tests/fixtures/projects/demo-purelib-with-build/setup.py", line 29, in <module>
    setup(**setup_kwargs)
  File "/usr/lib/python3.10/site-packages/setuptools/__init__.py", line 87, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.10/site-packages/setuptools/_distutils/core.py", line 122, in setup
    dist.parse_config_files()
  File "/usr/lib/python3.10/site-packages/setuptools/dist.py", line 854, in parse_config_files
    pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 54, in apply_configuration
    config = read_configuration(filepath, True, ignore_option_errors, dist)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 120, in read_configuration
    validate(subset, filepath)
  File "/usr/lib/python3.10/site-packages/setuptools/config/pyprojecttoml.py", line 43, in validate
    raise error from None
ValueError: invalid pyproject.toml config: `project`

Start at setuptools/config/pyprojecttoml.py, especially the validation path shown in the traceback, and reproduce the failure with the pdm-pep517 checkout and pytest command in the issue. Determine the expected handling of a pyproject.toml using pdm-specific keys, then verify the reported fixtures no longer fail validation.