GitHub actions fail due to flake8 not working anymore

[thjsal]

Seems that our GitHub Actions (continuous integration -tests in particular) are now failing because flake8 used for pytesting is not working anymore perhaps due to an inconsistent version of importlib-metadata package (error message is similar to that one here: PyCQA/flake8#1701):

Run # stop the build if there are Python syntax errors or undefined names
Traceback (most recent call last):
  File "/home/runner/micromamba-root/envs/xpsipy3/bin/flake8", line 10, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.[12](https://github.com/xpsi-group/xpsi/actions/runs/6755894386/job/18364788186#step:7:13)/site-packages/flake8/main/cli.py", line 22, in main
    app.run(argv)
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/main/application.py", line 375, in run
    self._run(argv)
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/main/application.py", line 363, in _run
    self.initialize(argv)
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/main/application.py", line 343, in initialize
    self.find_plugins(config_finder)
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/main/application.py", line [15](https://github.com/xpsi-group/xpsi/actions/runs/6755894386/job/18364788186#step:7:16)7, in find_plugins
    self.check_plugins = plugin_manager.Checkers(local_plugins.extension)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/plugins/manager.py", line 363, in __init__
    self.manager = PluginManager(
                   ^^^^^^^^^^^^^^
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/plugins/manager.py", line [24](https://github.com/xpsi-group/xpsi/actions/runs/6755894386/job/18364788186#step:7:25)3, in __init__
    self._load_entrypoint_plugins()
  File "/home/runner/micromamba-root/envs/xpsipy3/lib/python3.12/site-packages/flake8/plugins/manager.py", line [26](https://github.com/xpsi-group/xpsi/actions/runs/6755894386/job/18364788186#step:7:27)1, in _load_entrypoint_plugins
    eps = importlib_metadata.entry_points().get(self.namespace, ())
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'EntryPoints' object has no attribute 'get'
Error: Process completed with exit code 1.

I already tried to fix this by adding

    - name: Make sure metadata not too new for flake8
      run: |
        pip install "importlib-metadata<5.0"

into the .github/workflows/ci_tests.yml file in this pull request #338 , but that did not help (still the same error).