Install fails on Python 3.13

[svenmk]

When running pip install pycapnp in a fresh Python 3.13 environment, the installation fails with the following output:

pip install pycapnp                                                                                                                                                  Verfügbare Module werden g
Collecting pycapnp                                                                                                                                                                                                                                                                                     
  Downloading pycapnp-2.0.0.tar.gz (574 kB)                                                                                                                                                                                                                                                            
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 574.8/574.8 kB 6.4 MB/s eta 0:00:00
  Installing build dependencies ... done
  Getting requirements to build wheel ... done

[notice] A new release of pip is available: 24.2 -> 24.3.1
[notice] To update, run: python.exe -m pip install --upgrade pip
ERROR: Exception:
Traceback (most recent call last):
  File "D:\test\.venv\Lib\site-packages\pip\_internal\cli\base_command.py", line 105, in _run_wrapper
    status = _inner_run()
  File "D:\test\.venv\Lib\site-packages\pip\_internal\cli\base_command.py", line 96, in _inner_run
    return self.run(options, args)
           ~~~~~~~~^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\cli\req_command.py", line 67, in wrapper
    return func(self, options, args)
  File "D:\test\.venv\Lib\site-packages\pip\_internal\commands\install.py", line 379, in run
    requirement_set = resolver.resolve(
        reqs, check_supported_wheels=not options.target_dir
    )
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\resolver.py", line 95, in resolve
    result = self._result = resolver.resolve(
                            ~~~~~~~~~~~~~~~~^
        collected.requirements, max_rounds=limit_how_complex_resolution_can_be
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\resolvelib\resolvers.py", line 546, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\resolvelib\resolvers.py", line 397, in resolve
    self._add_to_criteria(self.state.criteria, r, parent=None)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\resolvelib\resolvers.py", line 173, in _add_to_criteria
    if not criterion.candidates:
           ^^^^^^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\resolvelib\structs.py", line 156, in __bool__
    return bool(self._sequence)
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\found_candidates.py", line 174, in __bool__
    return any(self)
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\found_candidates.py", line 162, in <genexpr>
    return (c for c in iterator if id(c) not in self._incompatible_ids)
                       ^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\found_candidates.py", line 53, in _iter_built
    candidate = func()
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\factory.py", line 186, in _make_candidate_from_link
    base: Optional[BaseCandidate] = self._make_base_candidate_from_link(
                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        link, template, name, version
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\factory.py", line 232, in _make_base_candidate_from_link
    self._link_candidate_cache[link] = LinkCandidate(
                                       ~~~~~~~~~~~~~^
        link,
        ^^^^^
    ...<3 lines>...
        version=version,
        ^^^^^^^^^^^^^^^^
    )
    ^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\candidates.py", line 303, in __init__
    super().__init__(
    ~~~~~~~~~~~~~~~~^
        link=link,
        ^^^^^^^^^^
    ...<4 lines>...
        version=version,
        ^^^^^^^^^^^^^^^^
    )
    ^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\candidates.py", line 158, in __init__
    self.dist = self._prepare()
                ~~~~~~~~~~~~~^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\candidates.py", line 235, in _prepare
    dist = self._prepare_distribution()
  File "D:\test\.venv\Lib\site-packages\pip\_internal\resolution\resolvelib\candidates.py", line 314, in _prepare_distribution
    return preparer.prepare_linked_requirement(self._ireq, parallel_builds=True)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\operations\prepare.py", line 527, in prepare_linked_requirement
    return self._prepare_linked_requirement(req, parallel_builds)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\operations\prepare.py", line 642, in _prepare_linked_requirement
    dist = _get_prepared_distribution(
        req,
    ...<3 lines>...
        self.check_build_deps,
    )
  File "D:\test\.venv\Lib\site-packages\pip\_internal\operations\prepare.py", line 72, in _get_prepared_distribution
    abstract_dist.prepare_distribution_metadata(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        finder, build_isolation, check_build_deps
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\distributions\sdist.py", line 56, in prepare_distribution_metadata
    self._install_build_reqs(finder)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\distributions\sdist.py", line 126, in _install_build_reqs
    build_reqs = self._get_build_requires_wheel()
  File "D:\test\.venv\Lib\site-packages\pip\_internal\distributions\sdist.py", line 103, in _get_build_requires_wheel
    return backend.get_requires_for_build_wheel()
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "D:\test\.venv\Lib\site-packages\pip\_internal\utils\misc.py", line 706, in get_requires_for_build_wheel
    return super().get_requires_for_build_wheel(config_settings=cs)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_impl.py", line 166, in get_requires_for_build_wheel
    return self._call_hook('get_requires_for_build_wheel', {
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        'config_settings': config_settings
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    })
    ^^
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_impl.py", line 321, in _call_hook
    raise BackendUnavailable(data.get('traceback', ''))
pip._vendor.pyproject_hooks._impl.BackendUnavailable: Traceback (most recent call last):
  File "D:\test\.venv\Lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 77, in _build_backend
    obj = import_module(mod_path)
  File "C:\Users\test\AppData\Local\Programs\Python\Python313\Lib\importlib\__init__.py", line 88, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1324, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'backend'