Replace setup.cfg for pyproject.toml

[santisoler]

Ditch the setup.cfg file and use only pyproject.toml. Move flake8 configurations to their own .flake8 file.

Relevant issues/PRs:

Related to fatiando/community#143

[leouieda]

Forgot to updated Dependente to v0.3 in the CI.

[leouieda]

Thanks for doing this!

[leouieda]

Maybe we wait for 2.0 to do this since it will remove the datasets module so a lot of that configuration will go away.