Migrate codespell configuration to pyproject.toml and get rid of setup.cfg

[DimitriPapadopoulos]

File setup.cfg had been created to add the configuration of codespell. Since codespell 2.2.2 the configuration can be read from pyproject.toml.

See [Description of PR].

TODO:

• [ ] Unit tests and/or doctests in docstrings
• [ ] tox -e py310 passes locally
• [ ] Docstrings and API docs for any new/modified user-facing classes and functions
• [x] Changes documented in docs/release.rst
• [ ] tox -e docs passes locally
• [x] GitHub Actions CI passes
• [x] Test coverage to 100% (Coveralls passes)