Flatpak-pip-generator: Cython required as a module

Next, I’m trying to get numpy included. Runtime 21.08.
I get the impression that Cython should be in the Runtime since 20.08. What am I missing?

Error:

Building wheels for collected packages: contourpy, numpy
  Running command Building wheel for contourpy (pyproject.toml)
  Building wheel for contourpy (pyproject.toml) ... done
  Created wheel for contourpy: filename=contourpy-1.1.1-cp38-cp38-linux_x86_64.whl size=19638792 sha256=9913f834f906567db320cf555312a03b84fba2bc399975a6ffb6c1897d4079d0
  Stored in directory: /home/jreynolds/.cache/pip/wheels/2f/d0/36/6cd5e4cca51aa4808e0e02b932665f9fabfa0dc5f1d0f46466
  Running command Building wheel for numpy (pyproject.toml)
  Running from numpy source directory.
  setup.py:67: DeprecationWarning:

    `numpy.distutils` is deprecated since NumPy 1.23.0, as a result
    of the deprecation of `distutils` itself. It will be removed for
    Python >= 3.12. For older Python versions it will remain present.
    It is recommended to use `setuptools < 60.0` for those Python versions.
    For more details, see:
      https://numpy.org/devdocs/reference/distutils_status_migration.html


    import numpy.distutils.command.sdist
  Traceback (most recent call last):
    File "setup.py", line 233, in generate_cython
      import Cython
  ModuleNotFoundError: No module named 'Cython'

  The above exception was the direct cause of the following exception:

  Traceback (most recent call last):
    File "/app/lib/python3.8/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
      main()
    File "/app/lib/python3.8/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/app/lib/python3.8/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 251, in build_wheel
      return _build_backend().build_wheel(wheel_directory, config_settings,
    File "/app/lib/python3.8/site-packages/setuptools/build_meta.py", line 221, in build_wheel
      return self._build_with_temp_dir(['bdist_wheel'], '.whl',
    File "/app/lib/python3.8/site-packages/setuptools/build_meta.py", line 207, in _build_with_temp_dir
      self.run_setup()
    File "/app/lib/python3.8/site-packages/setuptools/build_meta.py", line 258, in run_setup
      super(_BuildMetaLegacyBackend,
    File "/app/lib/python3.8/site-packages/setuptools/build_meta.py", line 150, in run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 479, in <module>
      setup_package()
    File "setup.py", line 458, in setup_package
      generate_cython()
    File "setup.py", line 240, in generate_cython
      raise OSError(msg) from e
  OSError: Cython needs to be installed in Python as a module
  error: subprocess-exited-with-error

  × Building wheel for numpy (pyproject.toml) 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.
  full command: /app/bin/python3.8 /app/lib/python3.8/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py build_wheel /tmp/tmplw5azoq8
  cwd: /tmp/pip-install-cp9nkz8d/numpy_9317fad06ac04a9393b1f7414ad5fa37
  Building wheel for numpy (pyproject.toml) ... error
  ERROR: Failed building wheel for numpy
Successfully built contourpy
Failed to build numpy
ERROR: Could not build wheels for numpy, which is required to install pyproject.toml-based projects
Error: module python3-contourpy: Child process exited with code 1

Manifest:

 $ less requirements.numpy.json 
{
    "name": "requirements.numpy",
    "buildsystem": "simple",
    "build-commands": [],
    "modules": [
        {
            "name": "python3-contourpy",
            "buildsystem": "simple",
            "build-commands": [
                "pip3 install --verbose --exists-action=i --no-index --find-links=\"file://${PWD}\" --prefix=${FLATPAK_DEST} \"contourpy\" --no-build-isolation"
            ],
            "sources": [
                {
                    "type": "file",
                    "url": "https://files.pythonhosted.org/packages/b1/7d/087ee4295e7580d3f7eb8a8a4e0ec8c7847e60f34135248ccf831cf5bbfc/contourpy-1.1.1.tar.gz",
                    "sha256": "96ba37c2e24b7212a77da85004c38e7c4d155d3e72a45eeaf22c1f03f607e8ab"
                },
                {
                    "type": "file",
                    "url": "https://files.pythonhosted.org/packages/a4/9b/027bec52c633f6556dba6b722d9a0befb40498b9ceddd29cbe67a45a127c/numpy-1.24.4.tar.gz",
                    "sha256": "80f5e3a4e498641401868df4208b74581206afbee7cf7b8329daae82676d9463"
                }
            ]
        },
        {
            "name": "python3-numpy",
            "buildsystem": "simple",
            "build-commands": [
                "pip3 install --verbose --exists-action=i --no-index --find-links=\"file://${PWD}\" --prefix=${FLATPAK_DEST} \"numpy\" --no-build-isolation"
            ],
            "sources": [
                {
                    "type": "file",
                    "url": "https://files.pythonhosted.org/packages/a4/9b/027bec52c633f6556dba6b722d9a0befb40498b9ceddd29cbe67a45a127c/numpy-1.24.4.tar.gz",
                    "sha256": "80f5e3a4e498641401868df4208b74581206afbee7cf7b8329daae82676d9463"
                }
            ]
        }
    ]
}

This runtime has been EOL for almost year. Flatpak likely told you so.