2.2.0 regression: All project installs with multiline descriptions are broken

[sigma67]

Since yesterday's release we are getting the following error on all our Python projects with multiline descriptions:

See /tmp/pdm-install-47973kyo.log for detailed debug log.
[BuildError]: Build backend raised error: Showing the last 10 lines of the build output:
    super().__init__(location, config_settings)
  File "/tmp/pdm-build-env-34cbu858-shared/lib/python3.10/site-packages/pdm/backend/base.py", line 99, in __init__
    self.config = Config.from_pyproject(self.location)
  File "/tmp/pdm-build-env-34cbu858-shared/lib/python3.10/site-packages/pdm/backend/config.py", line 79, in from_pyproject
    return cls(root, data)
  File "/tmp/pdm-build-env-34cbu858-shared/lib/python3.10/site-packages/pdm/backend/config.py", line 44, in __init__
    self.validate(data, root)
  File "/tmp/pdm-build-env-34cbu858-shared/lib/python3.10/site-packages/pdm/backend/config.py", line 65, in validate
    raise ValidationError(e.args[0], e.key) from e
pdm.backend.exceptions.ValidationError: The description must be a single line
WARNING: Add '-v' to see the detailed traceback

Affected descriptions look like this:

description = '''\
        A example Python package
        Lorem ipsum
    '''

This worked fine before 2.2.0

[sigma67]

The following multiline syntax seems unaffected:

description = """\
        A example Python package\
        Lorem ipsum\
    """

But only if backslashes are set at the end of the line - else same error.

[sigma67]

Root cause here: pypa/pyproject-metadata#76

[frostming]

That validation is added intentionally, and that is the specification. Please consider update your description to single-line.

[sigma67]

We will do so, but it is a pretty rough breaking change to be introduced over night.

It affects every package install as we don't have an upper bound on pdm-backend