[Python] How do I add system_platform specifiers to pyproject.toml?

I have a bare-bones pyproject.toml file in an empty directory:

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "test-virtualenvwrapper"
version = "0.1.0"

dependencies = [
'virtualenvwrapper ; system_platform == "linux"',
'virtualenvwrapper-win ; system_platform == "win32"',
]


When I run pip install -e . I get the following error:

(dev38) go|c:\srv\tmp\pyprojplat> pip install -e .
Looking in indexes: https://pypi.org/simple, http://pypi.norsktest.xyz
Obtaining file:///C:/srv/tmp/pyprojplat
Installing build dependencies ... done
Checking if build backend supports build_editable ... done
Getting requirements to build editable ... error
error: subprocess-exited-with-error

× Getting requirements to build editable did not run successfully.
│ exit code: 1
╰─> [48 lines of output]
configuration error: `project.dependencies[{data__dependencies_x}]` must be pep508
DESCRIPTION:
Project dependency specification according to PEP 508

GIVEN VALUE:
"virtualenvwrapper ; system_platform == \"linux\""

OFFENDING RULE: 'format'

DEFINITION:
{
"$id": "#/definitions/dependency",
"title": "Dependency",
"type": "string",
"format": "pep508"
}
Traceback (most recent call last):
File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 353, in <module>
main()
File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 335, in main
json_out['return_val'] = hook(**hook_input['kwargs'])
File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 132, in get_requires_for_build_editable
return hook(config_settings)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 447, in get_requires_for_build_editable
return self.get_requires_for_build_wheel(config_settings)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 338, in get_requires_for_build_wheel
return self._get_build_requires(config_settings, requirements=['wheel'])
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 320, in _get_build_requires
self.run_setup()
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 335, in run_setup
exec(code, locals())
File "<string>", line 1, in <module>
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\__init__.py", line 108, in setup
return distutils.core.setup(**attrs)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\_distutils\core.py", line 159, in setup
dist.parse_config_files()
File "C:\srv\venv\dev38\lib\site-packages\_virtualenv.py", line 21, in parse_config_files
result = old_parse_config_files(self, *args, **kwargs)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\dist.py", line 885, in parse_config_files
pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 62, in apply_configuration
config = read_configuration(filepath, True, ignore_option_errors, dist)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 126, in read_configuration
validate(subset, filepath)
File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 51, in validate
raise ValueError(f"{error}\n{summary}") from None
ValueError: invalid pyproject.toml config: `project.dependencies[{data__dependencies_x}]`.
configuration error: `project.dependencies[{data__dependencies_x}]` must be pep508
[end of output]

note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build editable did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

(dev38) go|c:\srv\tmp\pyprojplat>


I believe the system_platform specifiers are correct according to PEP508, so what am I doing wrong..?

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