Flake8 extension is incompatible with Flake8 6

[mdickinson]

This extension is incompatible with the recently-released flake8 6.0. Attempting to run flake8 on the extension gives the following output:

(ets-copyright-checker) mdickinson@mirzakhani ets-copyright-checker % python -m flake8
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/__main__.py", line 7, in <module>
    raise SystemExit(main())
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/main/cli.py", line 23, in main
    app.run(argv)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/main/application.py", line 198, in run
    self._run(argv)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/main/application.py", line 186, in _run
    self.initialize(argv)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/main/application.py", line 165, in initialize
    self.plugins, self.options = parse_args(argv)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/options/parse_args.py", line 51, in parse_args
    option_manager.register_plugins(plugins)
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/options/manager.py", line 259, in register_plugins
    add_options(self)
  File "/Users/mdickinson/Enthought/ETS/ets-copyright-checker/flake8_ets/copyright_header.py", line 189, in add_options
    option_manager.add_option(
  File "/Users/mdickinson/.venvs/ets-copyright-checker/lib/python3.10/site-packages/flake8/options/manager.py", line 281, in add_option
    self._current_group.add_argument(*option_args, **option_kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/argparse.py", line 1428, in add_argument
    raise ValueError('%r is not callable' % (type_func,))
ValueError: 'int' is not callable

A secondary problem is that our current CI doesn't pick up the issue, since it only works with flake8 from EDM. We should have a workflow that uses the latest flake8 from PyPI.

[mdickinson]

See PyCQA/flake8#1739 for the relevant upstream change.