PEP 621: Migrate config to pyproject.toml

[cclauss]

Migrate to pyproject.toml using ini2toml to do the file conversion and running validate-pyproject to validate results.

• Currently flake8 is not yet compatible with pyproject.toml so move its config to .flake8

[MartinThoma]

Thank you! I had to adjust the Makefile (replacing twine with flit): https://github.com/MartinThoma/flake8-simplify/commit/cfff4649ff3658c546c2ec445e33a4b1d9f71b4b