2.16.0 regression - KeyError: 'Requires-Python' crash when running pdm add in github actions on python 3.8 #2974

KeyError: 'Requires-Python'
i can't reproduce this locally, but it happens in github actions (windows-latest, ubuntu-latest and macos-latest) on python 3.8 but not 3.12. this bug seems to have been introduced in pdm 2.16.0
Make sure you run commands with -v flag before pasting the output.
Steps to reproduce
clone https://github.com/DetachHead/test/tree/0b2d5a592e30f2bfc76d8fddedf73b0ef17d6a4b
run the following commands in github actions on python 3.8:
./pw pdm install -v
./pw pdm add pytest==3.8 -v
Actual behavior
see github actions for the full output: https://github.com/DetachHead/test/actions/runs/9671949594/job/26683445102
 Traceback (most recent call last):
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/core.py", line 363, in main
    return core.main(args or sys.argv[1:])
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/core.py", line 281, in main
    raise cast(Exception, err).with_traceback(traceback) from None
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/core.py", line 276, in main
    self.handle(project, options)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/core.py", line 212, in handle
    command.handle(project, options)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/cli/commands/add.py", line 69, in handle
    self.do_add(
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/cli/commands/add.py", line 162, in do_add
    resolved = do_lock(
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/cli/actions.py", line 101, in do_lock
    mapping, dependencies = resolve(
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/resolver/core.py", line 39, in resolve
    result = resolver.resolve(requirements, max_rounds)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/resolvelib/resolvers.py", line 546, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/resolvelib/resolvers.py", line 426, in resolve
    name = min(unsatisfied_names, key=self._get_preference)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/resolvelib/resolvers.py", line 203, in _get_preference
    return self._p.get_preference(
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/resolver/providers.py", line 135, in get_preference
    deps = self.get_dependencies(candidate)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/resolver/providers.py", line 272, in get_dependencies
    deps, requires_python, _ = self.repository.get_dependencies(candidate)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/models/repositories.py", line 90, in get_dependencies
    requirements, requires_python, summary = getter(candidate)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/models/repositories.py", line 48, in wrapper
    result = func(self, candidate)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/models/repositories.py", line 270, in _get_dependencies_from_metadata
    deps = prepared.get_dependencies_from_metadata()
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/models/candidates.py", line 625, in get_dependencies_from_metadata
    return filter_requirements_with_extras(self.metadata.requires or [], extras)
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/pdm/models/candidates.py", line 618, in metadata
    self.candidate.requires_python = cast(str, result.metadata["Requires-Python"] or "")
  File "/home/runner/work/test/test/.pyprojectx/venvs/pdm-2ad7fbee887cffa1265ac100532e45d2-py3.8/lib/python3.8/site-packages/importlib_metadata/_adapters.py", line 54, in __getitem__
    raise KeyError(item)
KeyError: 'Requires-Python'
Expected behavior
no error
Environment Information
  "implementation_name": "cpython",
  "implementation_version": "3.8.18",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "6.5.0-1022-azure",
  "platform_system": "Linux",
  "platform_version": "#23~22.04.1-Ubuntu SMP Thu May  9 17:59:24 UTC 2024",
  "python_full_version": "3.8.18",
  "platform_python_implementation": "CPython",
  "python_version": "3.8",
  "sys_platform": "linux"
}