Use Dependente to capture dependencies on CI

[leouieda]

Dependente (https://github.com/fatiando/dependente) does what the code
in our tools folder did. Use it instead. Add the extra dependencies to
setup.cfg as install targets for pip so that they can be extracted by
dependente.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

Workflows are failing kind of randomly. Rerun again in a little while.