Replace TravisCI and Azure for GitHub Actions

[santisoler]

Add GitHub Actions workflows for checking style, testing and deploying
Harmonica, based on the workflows of Pooch. Remove TravisCI and Azure
configuration files. Split make check target in two: make black-check and
make flake8. Ditch Travis and Azure badges on README.rst. Add new GitHub
Actions badge. Tests are run on every matrix configuration: all OSs and all
Python versions. Documentation building is only run under Ubuntu with
Python 3.8.

Partially fixes fatiando/maintenance#1

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.