packaging: Centralize python related settings in pyproject.toml

The goal is to centralize python related settings in pyproject.toml and declutter the project root a bit.

TODO

• del pytest.ini
• del setup.cfg: flake8 does not (want to) support pyproject.toml, we use Flake8-pyproject to make up for it
• del mypi.ini + a bit of stubs cleanup / install

Not yet

• del tox.ini: tox's currently only support inlining the whole config in a legacy_tox_ini key which is not really satisfying, let's hope [WIP] Native TOML config gets merged soon.
• del setup.py: not sure how to handle data_files