PEP 621: Migrate more config into pyproject.toml

[cclauss]

Migrate configuration settings out of setup.cfg into pyproject.toml using ini2toml to do the file conversion and then run validate-pyproject to validate the results.

• flake8 is not currently compatible with pyproject.toml so migrate its config into .flake8.
• Comment out tool.setuptools.use-scm-version which does not pass validate-pyproject.

[ssbarnea]

Good move! I know it will upset few people due pip problems with editable installs but that should not be good enough reason to prevent it.

[cclauss]

Thx. Do you know of a good source of info on the editable installs issue?

[ssbarnea]

@cclauss That is the infamous bug pypa/pip#7953 but I learnt to avoid it.

Two workarounds: either use pyenv, and in that case you have user-level python, so you do not need to user --user option when installing stuff, or the more common approach, always use a virtualenv/venv as the editable problem does not happen with these.

PS. I will not make a release only with this patch as I do not find it worth a new release. OK?