Add a support of `--append-config`

[i-aki-y]

The flake8 supports --append-config option that overwrites a default config that is automatically loaded (or specified in --config). But the current pflake8 raises error when pyproject.toml set in --append-config:

# pflake8 --append-config pyproject.toml
Traceback (most recent call last):
  File "/opt/conda/bin/pflake8", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/flake8/main/cli.py", line 23, in main
    app.run(argv)
  File "/opt/conda/lib/python3.11/site-packages/flake8/main/application.py", line 198, in run
    self._run(argv)
  File "/opt/conda/lib/python3.11/site-packages/flake8/main/application.py", line 186, in _run
    self.initialize(argv)
  File "/opt/conda/lib/python3.11/site-packages/flake8/main/application.py", line 165, in initialize
    self.plugins, self.options = parse_args(argv)
                                 ^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/flake8/options/parse_args.py", line 29, in parse_args
    cfg, cfg_dir = config.load_config(
                   ^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/site-packages/flake8/options/config.py", line 90, in load_config
    if not cfg.read(filename, encoding="UTF-8"):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/conda/lib/python3.11/configparser.py", line 713, in read
    self._read(fp, filename)
  File "/opt/conda/lib/python3.11/site-packages/pflake8/__init__.py", line 35, in _read
    self.add_section(section)
  File "/opt/conda/lib/python3.11/configparser.py", line 674, in add_section
    raise DuplicateSectionError(section)
configparser.DuplicateSectionError: Section 'flake8' already exists

This PR allows to use pyproject.toml in --append-config. I also added simple test cases.

[csachs]

Thanks for the PR.