Replace TravisCI and Azure for GitHub Actions (#189)

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.