Move pytest, isort and mypy configuration from setup.cfg to pyproject.toml

[pquentin]

What the title says. Now that we have a pyproject.toml, we should strive to use it as our only configuration file. Annoyingly, flake8 refuses supporting pyproject.toml, but we should still migrate the others.

[hugovk]

Some useful tools for this:

• https://github.com/abravalheri/ini2toml to convert setup.cfg to pyproject.toml (but watch out for Create simpler "readme =" under [project] instead of [project.readme] section? abravalheri/ini2toml#50)

• https://github.com/abravalheri/validate-pyproject to check the output

• https://github.com/tox-dev/pyproject-fmt/ to apply a consistent format

The validator and formatter have a pre-commits hook in case you want to put them in .pre-commit-config.yaml.